Jugements rétrospectifs et prospectifs d'intervalles temporels chez les personnes souffrant d'insomnie et les bons dormeurs

U.U

3

LES PERSONNES SOUFFRANT D'INSOMNIE ET LES BONS DORMEURS

Mémoire présenté

à la Faculté des Études Supérieures de l'Université Laval

pour l'obtention

du grade de maître en psychologie (M.Ps.)

École de Psychologie

FACULTÉ DES SCIENCES SOCIALES UNIVERSITÉ LAVAL

AOÛT 2001

Résumé

Des écarts considérables existent lors de la comparaison des paramètres de sommeil objectifs et subjectifs chez les personnes souffrant d'insomnie: la sous-estimation du temps total de sommeil, du nombre et de la durée des éveils, ainsi que la surestimation de la latence de sommeil. Les bons dormeurs manifestent aussi ces erreurs, mais à un degré beaucoup moindre. Il est possible que les personnes souffrant d'insomnie manifestent un déficit au niveau de la perception du temps. Une hypothèse alternative serait que ces personnes perçoivent mal leur état de sommeil. La présente étude vise à comparer la perception du temps des personnes souffrant d'insomnie et des bons dormeurs. Trente bons dormeurs et trente personnes souffrant d'insomnie se sont soumis à quatre tâches d'estimation temporelle dans une condition non liée au sommeil (attente dans une chambre) et dans une condition liée au sommeil (sieste dans l'après-midi). Les résultats suggèrent que les personnes souffrant d'insomnie soient moins précises que les bons dormeurs pour les intervalles longs (12,5 minutes). De plus, ce déficit serait présent dans des contextes lié et non lié au sommeil lorsqu'elles estiment le temps rétrospectivement, mais seulement dans des situations liées au sommeil prospectivement. Ces résultats soulignent également l'importance de considérer les effets possibles de pratique et d'ordre de présentation lorsque la perception du temps est évaluée. En conclusion, il est possible que les personnes souffrant d'insomnie soient enclines à surévaluer leur latence de sommeil parce que leur perception du temps est imprécise en situation liée au sommeil. Toutefois, des limites méthodologiques soulignent le besoin de répliquer ces résultats au cours d'études futures.

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Abstract

There are large discrepancies when comparing subjective and objective sleep parameters in insomniacs: underestimation of total sleep time, number and length of wake after sleep onset and overestimation of sleep latency. Good sleepers display these errors to a lesser extent. It is unclear whether insomniacs have an impairment in time perception or misperceive their sleep state. The present study aimed at comparing the time perception of insomniacs and good sleepers in sleep-related and sleep-unrelated settings. Thirty good sleepers and thirty insomniacs underwent four temporal estimation tasks in a sleep- unrelated condition (waiting in a room) and in a sleep-related condition (during an afternoon nap). The results suggest that insomniacs might be less accurate than good sleepers when estimating long intervals. Furthermore, this impairment would be present in sleep-unrelated and sleep-related contexts when they estimate time retrospectively, but only in sleep-related situations when estimating prospectively. Also, the results

emphasize the importance of considering practice and order effects when evaluating time perception of short and medium intervals. In conclusion, insomniacs may overestimate their sleep latency because their time perception is inaccurate in sleep-related situations. However, methodological flaws accentuate the need to replicate these results in future studies.

Avant-propos

Je voudrais remercier toutes les personnes qui ont permis, de près ou de loin, la réalisation de ce mémoire :

Merci à mon directeur de recherche, Monsieur Charles M. Morin, pour ses précieux conseils et pour avoir cru en mes capacités.

Merci à mes collègues, particulièrement Mélanie, Marie-Christine, Annie et Célyne, pour les partages, autant d'expertise que d'angoisse, les fous rires et le soutien sans faille.

Merci à mes amis, Philippe, William, Jonathan et Marc, pour les encouragements, la présence constante, les bras ouverts et la merveilleuse complicité.

Merci à ma mère, Madeleine, d'être là simplement lorsque le besoin est.

Enfin, merci à ma précieuse compagne de vie, Marie-Jo. Merci pour ta patience, ton support, ton sourire et tes yeux où je peux puiser mon réconfort. Je t'aime gros comme le ciel. Pour toujours.

Page

RÉSUMÉ... ii

ABSTRACT ... iii

AVANT-PROPOS... iv

TABLE DES MATIÈRES... v

LISTE DES TABLEAUX... viii

LISTE DES FIGURES... ix

CHAPITRE I : INTRODUCTION 1.1 Introduction... 2

1.2 Notions de base du sommeil... 2

1.3 Insomnie : définition, prevalence et épidémiologie... 3

1.4 Comparaison entre les paramètres de sommeil subjectifs et objectifs... 4

1.5 Perception du temps : modèles et mesures... 5

1.6 Pertinence et implications de l'étude... 8

1.7 Contenu du mémoire... 10

CHAPITRE II : JUGEMENTS RÉTROSPECTIFS ET PROSPECTIFS D'INTERVALLES TEMPORELS CHEZ LES PERSONNES SOUFFRANT D'INSOMNIE ET LES BONS DORMEURS (ARTICLE) 2.1 Page titre... 12 2.2 Résumé... 13 2.3 Front page... 14 2.4 Abstract... 15 16 2.5 Introduction

Page

2.5.1 Perceiving time : paradigms and methods of measurement... 19

2.5.2 Misperceiving sleep onset... 21

2.5.3 Depression, anxiety, and medication : their effects on sleep and time perception... 23

2.5.4 Summary and aim of the present study... 24

2.6 Method... 26 2.6.1 Participants... 26 2.6.2 Sleep evaluation... 27 2.6.3 Psychological evaluation... 28 2.6.4 Procedure... 29 2.6.5 Tasks... 30 2.7 Results... 31 2.8 Discussion... 35 2.9 References... 41 2.10 Tables... 51 2.11 Figure Captions... 58 2.12 Figures... 59 CHAPITRE ΠΙ : CONCLUSION 3.1 Conclusion... 61 3.2 Principaux résultats... 61 3.3 Limites de l'étude... 63

3.4 Implications et recherches futures... 64

TABLEAUX (INTRODUCTION ET CONCLUSION)... 73 ANNEXES... 78

Page INTRODUCTION

Tableau 1 Prévalence et facteurs associés à l'insomnie... 73 ARTICLE

Table 1 Procedure and verbatim descriptions (sleep-unrelated condition) .... 51 Table 2 Procedure and verbatim descriptions (sleep-related condition)... 52 Table 3 Means and standard deviations for subjective sleep parameters... 53 Table 4 Means and standard deviations for Beck Depression Inventory

(BDI), Beck Anxiety Inventory (BAI), and Sleep Impairment

Index (SB)... 54 Table 5 Means and standard deviations for ratios of temporal subjective

estimates over the objective duration for good sleepers and

insomniacs... 55 Table 6 Means and standard deviations for ratios of temporal subjective

estimates over the objective duration of participants who began with the sleep-unrelated condition and participants who began

with the sleep-related condition... 56 Table 7 Summary of the four repeated measures ANOVAs of log

transformed ratios with two between-group variables (group and

Page ARTICLE

59 Figure 1

1.1 Introduction

Ce premier chapitre vise à introduire de façon générale le thème du présent mémoire. Des notions de base du sommeil seront tout d'abord abordées. Suivra une description de Γinsomnie et des facteurs qui y sont associés. Également, les écarts typiques entre les paramètres de sommeil objectifs et subjectifs observés chez les

personnes souffrant d'insomnie seront illustrés. Par la suite, le thème de la perception du temps sera brièvement introduit. Enfin, la pertinence et les implications de l'étude ainsi que le contenu du mémoire seront décrits.

1.2 Notions de base du sommeil

Selon Carskadon et Dement (1989), le sommeil est, d'un point de vue strictement comportemental, un état réversible de désengagement perceptuel et d'absence de réponse envers l'environnement. Le sommeil se divise grossièrement en deux phases, sommeil REM (paradoxal) et NREM (sommeil lent). La phase de sommeil lent est caractérisée par des ondes cérébrales lentes et un ralentissement de plusieurs fonctions physiologiques (rythmes cardiaque et respiratoire, etc.). Cette phase se divise en quatre stades, illustrant la progression du sommeil en profondeur. Le sommeil paradoxal, quant à lui, se

distingue par un taux élevé d'activation cérébrale, une atonie musculaire et des accès épisodiques de mouvements rapides des yeux.

Le moment précis de l'endormissement semble difficile à identifier précisément. L'éveil paraît faire place au sommeil de façon graduelle. Différentes fonctions

perceptuelles et cognitives, telles la perception sensorielle, la mémoire, la conscience de soi, la continuité de la pensée logique et la latence de réponse à un stimulus, perdent de leur acuité au fur et à mesure que le sommeil s'installe (Carskadon & Dement, 1989).

Le patron et la longueur d'une nuit de sommeil sont très variables d'un individu à l'autre. Il est cependant possible de dégager un patron général pour le sommeil normal d'un adulte: la nuit débute par une phase de sommeil lent, une phase paradoxale se produit après environ 90 minutes et, par la suite, ces deux phases alternent de façon cyclique pendant la nuit selon des périodes d'environ 90 minutes (Morin, 1993). La plupart des jeunes adultes rapportent dormir approximativement 7,5 heures les nuits de semaine et un peu plus longtemps, soit 8,5 heures, les nuits de fins de semaine

(Carskadon & Dement, 1989).

Chez les personnes âgées, le patron de sommeil est quelque peu différent. Bliwise (1989) rapporte une diminution de l'efficacité du sommeil et une augmentation du

pourcentage de sommeil en stade 1. Il mentionne également que le changement lié à l'âge le plus souvent rapporté est une diminution du sommeil profond, c'est-à-dire en stades 3 et 4. Le cycle éveil-sommeil se modifie également, diminuant la durée du sommeil nocturne et augmentant la prédisposition à dormir pendant !ajournée.

1.3 Insomnie: définition, prévalence et épidémiologie

L'insomnie est la plainte subjective d'insuffisance ou de caractère inadéquat du sommeil. Le moment de la perturbation détermine trois types généraux d'insomnie. Une latence d'endormissement retardée est caractéristique de l'insomnie initiale. La continuité défaillante du sommeil, quant à elle, est propre à Yinsomnie de maintien. Enfin,

l'insomnie terminale se distingue par des éveils matinaux prématurés. Il peut évidemment exister un certain chevauchement entre ces trois catégories; le type d'insomnie est alors mixte.

Le tableau 1 résume une série d'études récente sur la prévalence et les facteurs associés à l'insomnie. La prévalence de l'insomnie se situerait entre 10 et 35% de la population générale. La grande étendue de cet estimé reflète différentes méthodologies d'évaluation des difficultés de sommeil et différents critères diagnostiques de l'insomnie. En effet, certaines investigations reposent sur une question générale sur la présence ou l'absence de difficultés de sommeil, alors que d'autres passent en revue tous les critères diagnostiques de l'insomnie un à un. Les facteurs les plus souvent associés aux difficultés de sommeil sont (a) le fait d'être une femme; (b) le fait d'être âgé; (c) l'humeur

dépressive; (d) les troubles d'anxiété; et (e) la présence d'un trouble physique et/ou mental. Il est important de souligner que ces facteurs sont corrélationnels et non causaux. En ce qui concerne l'anxiété et la dépression en lien avec l'insomnie, par exemple, il est souvent difficile de départager lequel trouble est survenu en premier, lequel constitue la cause et lequel l'effet. Les difficultés de sommeil sont également associées avec le fait d'être âgé. Toutefois, comme nous l'avons mentionné plus tôt, le patron de sommeil semble se modifier avec l'âge. La constatation d'une prévalence plus grande de difficultés de sommeil chez les personnes âgées peut refléter en partie ce phénomène. Enfin, la présence d'un trouble physique est fortement corrélée avec la présence d'insomnie, puisque le sommeil est facilement perturbé par la douleur ou l'inconfort.

Insérer Tableau 1

1.4 Comparaison entre les paramètres de sommeil subjectifs et objectifs La comparaison des mesures objectives du sommeil (données

polysomnographiques; PSG) et des estimés subjectifs révèle que les personnes souffrant d'insomnie sont souvent imprécises lorsqu'elles estiment leurs propres paramètres de

sommeil. Les erreurs typiquement observées sont : (a) la sous-estimation du temps total de sommeil; (b) la sous-estimation du nombre et de la durée des éveils nocturnes; et (c) la surestimation de la latence de sommeil. Ces erreurs ont été rapportées de façon constante par une série d'auteurs (Bonnet, 1990; Borkovec, Lane & VanOot, 1981; Carskadon et al.,

1976; Coates et al., 1982; Coates et al., 1983; Edinger & Fins, 1995; Frankel, Coursey, Buchbinder & Snyder, 1976; Hédouin, Lagrange & Leroy, 1981; Vanable, Aikens, Tadimeti, Caruana-Montaldo & Mendelson, 2000). Par exemple, Vanable et ses collaborateurs (2000) ont compilé les ratios des latences de sommeil subjectives par rapport aux latences objectives chez des personnes souffrant d’insomnie. Ils ont obtenu des ratios variant de 115,0 à 131,8. Autrement dit, la latence de sommeil subjective estimée par ces personnes était de 1,15 à 1,32 fois la valeur de la latence objective.

Bien qu’ils ne soient pas exempts de ce type d’erreurs, les bons dormeurs semblent produire des estimations beaucoup plus précises. Baker, Maloney et Driver (1999) ont rapporté une légère surestimation de la latence de sommeil de l’ordre de cinq à dix minutes et une moyenne de quatre éveils nocturnes sous-estimés. Il semblerait donc que les divergences entre données subjectives et objectives soient plus prononcées et plus caractéristiques chez les personnes souffrant d’insomnie que chez les bons dormeurs.

Ces observations ont soulevé l’hypothèse d’un déficit au niveau de l’estimation temporelle chez les gens souffrant d’insomnie. Ce groupe de personnes particulier souffre peut-être d’un certain trouble de la perception du passage du temps, les amenant à surestimer la latence de sommeil et à sous-estimer la durée totale du sommeil.

Plusieurs facteurs influencent la perception du temps. Block (1990) les regroupe en quatre groupes : (a) les caractéristiques de la personne qui produit l’estimé; (b) le contenu de l’intervalle de temps à estimer; (c) les activités pendant la période de temps; et (d) la durée de l’intervalle à estimer. Hicks, Miller et Kinsboume (1976; cités par Zakay, 1990) considèrent plutôt les quatre suivants : (a) la méthode de mesure de la perception du temps; (b) la durée de l’intervalle; (c) la nature des procédés cognitifs requis par le participant pendant l’intervalle (selon que cet intervalle est «vide» ou «rempli»); et (d) la nature du paradigme d’évaluation (rétrospectif ou prospectif). Nichelli (1996) ajoute que (a) les stratégies employées par le participant (voir aussi Grondin, Meilleur-Wells & Lachance, 1999); (b) la segmentation de l’intervalle; et (c) l’ordre de présentation des stimuli peuvent aussi influencer l’estimation. Enfin, la modalité requise par les marqueurs délimitant l’intervalle (auditive ou visuelle) a également un effet sur la perception du temps. Cette multitude de facteurs est rarement prise en considération dans les études comparant les données de sommeil subjectives et objectives. Pourtant, l’estimation d’un paramètre de sommeil est en fait une estimation temporelle. Par exemple, si la durée de l’intervalle en influence l’estimation

subséquente, il est possible que les personnes souffrant d’insomnie estiment leur latence de sommeil de façon moins précise simplement parce que cette dernière est plus longue chez ces gens.

Un modèle largement utilisé par les spécialistes de la perception du temps est celui de l’horloge interne (Block, 1990; Grondin, 1998; Kladopoulos, Brown, Hemmes & Cabeza de Vaca, 1998; Treisman, Cook, Naish & MacCrone, 1994). Dans sa version la plus simple, l’horloge interne serait un mécanisme composé de deux parties principales, soit un émetteur et un accumulateur. L’accumulation de pulsations générées par

l’émetteur est la base de l’expérience du passage du temps. Plus nombreuses sont les pulsations accumulées, plus longue sera la durée perçue. La variabilité dans l’émission des pulsations est considérée comme étant la principale source d’erreur. Donc, si les personnes souffrant d’insomnie démontrent un véritable déficit dans leur perception temporelle, il sera possible de supposer que le trouble se situe au niveau de l’émetteur.

L’évaluation de la perception du temps peut se faire selon deux paradigmes. Les jugements temporels peuvent être prospectifs ou rétrospectifs. En paradigme prospectif,

le/la participant(e) est conscient(e) qu’il/elle aura à produire une estimation d’un intervalle donné. Cette estimation est aussi appelée «durée expérimentée». En paradigme rétrospectif, la durée estimée est «rappelée», c’est-à-dire que le/la

participant(e) ne sait pas qu’il/elle aura à estimer une durée jusqu’à ce que l’intervalle soit terminé. Des processus cognitifs différents sous-tendent les durées «expérimentée» et «rappelée». Une méta-analyse récente effectuée par Block et Zakay (1997) souligne le fait que des modèles attentionnels sont nécessaires pour expliquer les jugements

prospectifs, alors que les jugements rétrospectifs sont mieux expliqués avec des modèles basés sur la mémoire. De ce fait, en jugement prospectif, le temps paraît passer plus rapidement si la personne est engagée dans des activités requérant son attention. Par exemple, une heure de conversation intéressante avec un bon ami semblera passer plus rapidement qu'une heure à essayer en vain de s'endormir. Cet effet est renversé pour les jugements rétrospectifs : une période remplie d’événements intéressants sera estimée

(rappelée) plus longue qu’une période qui en serait dénuée (Nichelli, 1996). Autrement dit, se rappeler d'une période d'une heure remplie d'appels téléphoniques, de réponses à des courriels et de rencontres avec des supérieurs mènera à un estimé plus long que se rappeler d'une heure passée à suivre un cours ennuyeux.

Il existe plusieurs méthodes pour mesurer la perception du temps. Bindra et Waksberg, en 1956, en ont rapporté quatre principales : (a) l’estimation verbale, où un intervalle délimité par !’expérimentateur est estimé verbalement en unités

conventionnelles (secondes, minutes, heures, etc.); (b) la production, où le participant doit marquer le début de la fin d'un intervalle donné verbalement (et en unités

conventionnelles) par Γ expérimentateur; (c) la reproduction, où le participant doit reproduire une durée d’abord présentée par 1 ’expérimentateur; et (d) la comparaison, où le participant doit juger lequel de deux intervalles présentés est le plus long (ou le plus court). Allan (1979) mentionne qu’il n’existe pas de méthode clairement supérieure à une autre et que, de plus, les différentes méthodes ne sont pas toujours corrélées.

Zakay (1990) a décrit en détails la méthode de l’estimation verbale, méthode utilisée dans la présente étude. L’estimation verbale est une traduction de la perception subjective du temps en unités conventionnelles. Cette méthode est encline aux biais de rapporter les estimés en chiffres ronds (multiples de cinq) (voir aussi Nichelli, 1996). Elle est également encline aux biais de représentativité et de disponibilité, en ce sens que le participant peut produire son estimation sur la base de la similarité de la durée à une tâche dont la durée est connue. Par exemple, le participant peut se dire qu’il aurait eu le temps de faire ses 15 minutes de course quotidienne pendant l’intervalle, et ainsi estimer l’intervalle à 15 minutes.

1.6 Pertinence et implications de l’étude

Cette étude vise à comparer la perception du temps des personnes souffrant d’insomnie à celle de participants contrôles. La contribution nouvelle qu’elle apporte est qu’une comparaison se fera également entre la perception du temps en condition liée au

sommeil (une sieste dans l'après-midi) et non liée au sommeil (l'attente dans une

chambre). De plus, !’établissement d’intervalles fixes à estimer permettra d’observer si la surestimation de la latence de sommeil chez les gens souffrant d’insomnie n’est qu’un artefact dû au fait que la latence de sommeil est plus longue chez ces personnes. Il sera ainsi possible de déterminer si, premièrement, les gens souffrant d’insomnie perçoivent le temps différemment des bons dormeurs et si, le cas échéant, cette différence se manifeste seulement dans une situation de demande d’endormissement (liée au sommeil) ou

indifféremment daris les conditions liée et non liée au sommeil.

Les implications de cette recherche sont nombreuses. Au plan théorique, !'observation d'une perception du temps inexacte chez les gens souffrant d'insomnie soulève certaines questions. Est-il possible de faire un lien entre un éventuel déficit de perception temporelle et un débalancement du cycle éveil-sommeil sous-jacent à l'insomnie? Peut-être ces deux phénomènes sont-ils eux-mêmes sous-jacents à la suractivation physiologique et cognitive souvent observée chez les personnes souffrant d'insomnie (Bonnet & Arand, 1994; Borkovec, 1982; Perlis, Smith, Andrews, Orff & Giles, 2001). Dans ce cas, l'insomnie serait davantage un symptôme d'un trouble plus général plutôt qu'un syndrome propre. L'hypothèse stipulant que la mauvaise perception temporelle des gens souffrant d'insomnie sous-tend une suractivation physiologique et cognitive est davantage discutée dans le prochain chapitre.

Au niveau pratique, l'écart entre les évaluations objectives et subjectives des paramètres de sommeil évoque des considérations psychométriques. Quelle évaluation du sommeil (subjective ou objective) offre la meilleure validité en situation de recherche? Est-ce la même qu'en clinique? Afin que cet écart entre les deux types de mesures

écart à un déficit caractéristique des personnes aux prises avec des difficultés de sommeil, on stipule que les mesures polysomnographiques reflètent la réalité et que ce groupe de personnes possède un déficit perceptuel qui l'empêche de se représenter cette réalité de

façon exacte.

Enfin, au niveau clinique, des données sur la perception du temps des gens souffrant d'insomnie sont essentielles. En l'absence d'appareillage polysomnographique coûteux, le clinicien n'a souvent accès qu'à l'évaluation subjective du patient de son propre sommeil. S'il est démontré qu'une personne souffrant d'insomnie ne peut fournir d'estimés temporels relativement exacts, le clinicien devra interpréter l'évaluation

subjective du sommeil avec précaution. Cette interprétation aura peut-être plus de valeur au niveau qualitatif (e.g. ce patient considère avoir une qualité de sommeil médiocre) que quantitatif (e.g. ce patient dit prendre deux heures pour s'endormir).

1.7 Contenu du mémoire

La première section du mémoire constitue une introduction générale à la

problématique abordée. L’article rapportant les détails de l’étude effectuée compose le chapitre deux. Cet article représente le corps du mémoire et est rédigé en respectant les normes de publication de Y American Psychological Association (APA). Les différentes étapes de l’expérimentation (recrutement des participants, évaluation psychologique, évaluation du sommeil, tâches d’estimation temporelle) ont été réalisées au cours des années 2000 et 2001 par l’auteure du présent mémoire. Enfin, la conclusion générale du mémoire - décrivant les principaux résultats obtenus, les limites et les implications de l’étude ainsi que des suggestions pour les recherches futures - se retrouve au chapitre trois.

Jugements rétrospectifs et prospectifs d’intervalles temporels chez les personnes souffrant d’insomnie et les bons dormeurs

chez les personnes souffrant d’insomnie et les bons dormeurs

Geneviève Belleville et Charles M. Morin

Université Laval, Sainte-Foy, Canada

Des écarts considérables existent lors de la comparaison des paramètres de sommeil objectifs et subjectifs chez les personnes souffrant d'insomnie: la sous-estimation du temps total de sommeil, du nombre et de la durée des éveils, ainsi que la surestimation de la latence de sommeil. Les bons dormeurs manifestent aussi ces erreurs, mais à un degré beaucoup moindre. Il est possible que les personnes souffrant d'insomnie manifestent un certain déficit au niveau de la perception du temps. Une hypothèse alternative serait que ces personnes perçoivent mal leur état de sommeil. La présente étude vise à comparer la perception du temps des personnes souffrant d'insomnie et des bons dormeurs. Trente bons dormeurs et trente personnes souffrant d'insomnie se sont soumis à quatre tâches d'estimation temporelle dans une condition non liée au sommeil (attente dans une chambre) et dans une condition liée au sommeil (sieste dans l'après-midi). Les résultats suggèrent que les personnes souffrant d'insomnie soient moins précises que les bons dormeurs pour les intervalles longs. De plus, ce déficit serait présent dans des contextes lié et non lié au sommeil lorsqu'elles estiment le temps rétrospectivement, mais seulement dans des situations liées au sommeil prospectivement. Ces résultats soulignent également l'importance de considérer les effets possibles de pratique et d'ordre de présentation lorsque la perception du temps est évaluée. En conclusion, il est possible que les

personnes souffrant d'insomnie soient enclines à surévaluer leur latence de sommeil parce que leur perception du temps est imprécise en situation liée au sommeil. Toutefois, des limites méthodologiques soulignent le besoin de répliquer ces résultats au cours d'études futures.

Retrospective and prospective estimates of time intervals in insomniacs and good sleepers

Geneviève Belleville and Charles M. Morin

Université Laval, Sainte-Foy, Canada

There are large discrepancies when comparing subjective and objective sleep parameters in insomniacs: underestimation of total sleep time, number and length of wake after sleep onset and overestimation of sleep latency. Good sleepers display these errors to a lesser extent. It is unclear whether insomniacs have an impairment in time perception or misperceive their sleep state. The present study aimed at comparing the time perception of insomniacs and good sleepers in sleep-related and sleep-unrelated settings. Thirty good sleepers and thirty insomniacs underwent four temporal estimation tasks in a sleep- unrelated condition (waiting in a room) and in a sleep-related condition (during an afternoon nap). The results suggest that insomniacs might be less accurate than good sleepers when estimating long intervals. Furthermore, this impairment would be present in sleep-unrelated and sleep-related contexts when they estimate time retrospectively, but only in sleep-related situations when estimating prospectively. Also, the results

emphasize the importance of considering practice and order effects when evaluating time perception of short and medium intervals. In conclusion, insomniacs may overestimate their sleep latency because their time perception is inaccurate in sleep-related situations. However, methodological flaws accentuate the need to replicate these results in future studies.

Are insomniacs poor judges of time?

Retrospective and prospective estimates of time intervals in insomniacs and good sleepers

Research and clinical data have consistently shown that insomniacs are not accurate when estimating their sleep parameters. When subjective and objective

(polysomnographic - PSG) sleep measures are compared, insomniacs underestimate their total sleep time, the number and length of wake after sleep onset, and they overestimate their sleep latency (Carskadon et ah, 1976; Coates et ah, 1982; Coates et ah, 1983; Frankel, Coursey, Buchbinder & Snyder, 1976; Hédouin, Lagrange & Leroy, 1981). Good sleepers are not exempt from these kinds of errors, but they seem to display them to a lesser extent. Research showed sleep latency overestimation of about five minutes in good sleepers (Bonnet & Moore, 1982). Moore (1981; cited by Downey and Bonnet,

1992) indicated that subjective insomniacs, i.e. patients with a complaint of insomnia but whose objective sleep measures are similar to those of good sleepers, overestimated nocturnal sleep latency by an average of 42.8 minutes. In sharp contrast, participants without sleep complaints overestimated it by an average of only 1.4 minutes. Baker, Maloney and Driver (1999) have observed mean sleep latency overestimations from five to ten minutes in good sleepers over a three night evaluation. These individuals also significantly underestimated their number of awakenings per night, i.e. a mean of four awakenings were not perceived. In elderly people, the sleep latency overestimation and the underestimation of total sleep time, frequency and length of wake are also present (Libman, Creti, Levy Brender & Fichten, 1997), but the magnitude of the discrepancy do not differ among good sleepers, insomniacs without medication or insomniacs taking benzodiazepines (Jobidon, Jodouin & Morin, 1999).

The discrepancy between objective and subjective sleep measures in insomniacs has led to a variety of explanations from sleep specialists. Some authors hypothesised that physiological or cognitive hyperarousal is responsible for the poor time estimation in insomniacs (Bonnet & Arand, 1994; Borkovec, 1982). Both hypotheses suggest that hyperarousal at sleep onset, implicating high metabolic rate in the first case (see also Bonnet & Arand, 1997) or high number of cognitive intrusions in the second (see also Gross & Borkovec, 1982; Kuisk, Bertelson & Walsh, 1989; and Van Egeren, Haynes, Franzen & Hamilton, 1983), produces a state opposite to that necessary for sleep to occur. This hyperarousal state would also cause the perception of time running faster, hence the overestimation of sleep latency. Indeed, it has been demonstrated that primary insomnia is associated with increased high frequency activity during sleep (Perils, Smith, Andrews, Orff & Giles, 2001).

Insomniacs have more difficulties than good sleepers to disengage sensory and cognitive processes during wake-sleep transitions (Perils, Giles, Mendelson, Bootzin & Wyatt, 1997). This can blur the distinction between wake and sleep. According to Perils et al. (1997), an intact ability to encode and retrieve information in insomniacs at sleep onset could influence the perception of time. However, why would this inact ability cause insomniacs to misperceive time? If memory is involved in retrospective time estimation of long intervals (as we shall discuss in a latter section) and if it is impaired in good sleepers at sleep onset, then why are good sleepers better estimators than

insomniacs?

Two additional hypotheses warrant consideration. It could be that insomniacs show a certain impairment in their time perception, either in general (i.e. even during the day) or in a sleep-related context. It could also be that insomniacs misperceive their

actual sleep state. If they think they are awake while PSG indicates sleep, this would explain why their subjective sleep parameters do not correspond to the objective ones. Borkovec, Lane and VanOot (1981) found that when awakened from stage 2, insomniacs are less likely to report being asleep than good sleepers. This led them to hypothesise that insomniacs might need to be in stage 2 for a longer period for them to perceive they are asleep. Bonnet (1990) also believed that insomniacs do not necessarily misperceive time per se, but that they do have difficulties perceiving their sleep state. Chervin and Guilleminault (1996) observed a similar phenomenon in people suffering from excessive daytime sleepiness.

Some authors have also raised the possibility that time perception is impaired in insomniacs (Mendelson, James, Garnett, Sack & Rosenthal, 1986). They noticed that, after forced awakenings, insomniacs and good sleepers went back to sleep with equal facility, but their perception of subsequent sleep intervals differed: insomniacs

underestimated the quantity of sleep while good sleepers were fairly accurate judges of sleep duration. It is possible, then, that insomniacs misperceived their sleep state. However, the authors also reported that insomniacs overestimated the duration between two awakenings. The question therefore remains whether insomniacs are poor judges of time.

To evaluate the time perception of insomniacs and good sleepers, ratios of

subjective / objective latency of twelve male insomniacs have been compared to ratios of twelve control participants (Moore, Bonnet, Warm & Kramer, 1980). Significant group differences observed for estimates made in the morning and at sleep onset suggest that insomniacs may have perceptual difficulties in judging temporal duration. This team carried on another study investigating the generality of insomniacs’ overestimation of

temporal intervals, i.e. whether insomniacs misperceive time only in the unpleasant situation when they have to try to fall asleep (a sleep-related condition) or if this impairment is displayed in all situations (sleep-related and sleep-unrelated conditions) (Moore, Bonnet & Warm, 1982). Their results were not significant, i.e. insomniacs did not produce temporal estimates different from control participants neither in sleep-related intervals (estimating the duration of a 19 minute afternoon nap) nor in sleep-unrelated intervals (19 minute, 5 second and 35 second intervals). However, having only twelve subjects in each condition, it is possible that their analysis lacked the power necessary to find significant differences.

If insomniacs are unable to estimate time accurately, one should expect that they would display some level of impairment in certain cognitive functions. However, no clear cognitive deficit has been identified in insomniacs that could account for an

inaccurate time perception (Adam, Tomeny & Oswald, 1986; Broman, Lundh, Aleman & Hetta, 1992; Hauri, 1997; Hyyppae, Kronholm & Mattar, 1991; Mendelson, Garnett, Gillin & Weingartner, 1984; Schneider-Helmert, 1987; Sugerman, Stem & Walsh, 1985). In condition of sleep deprivation, recent studies have demonstrated that a specific

reduction of slow-wave sleep induced alterations in cognitive performance among insomniacs (Jurado, Luna-Villegas & Buela-Casal, 1989). However, this effect needs to be replicated. Moreover, it is not consistently observed among good sleepers (Crenshaw & Edinger, 1999; Edinger, Glenn, Bastian & Marsh, 2000).

Perceiving time : paradigms and methods of measurement

Two paradigms exist in the time perception literature. Judgements of temporal intervals can be made prospectively or retrospectively. In a prospective paradigm, the subject is aware that he/she will have to produce an estimation of a given interval.

also called «experienced duration». In a retrospective paradigm, the estimated duration is remembered, i.e. the subject does not know that he/she will have to estimate the duration until the interval is over. Different cognitive processes underlie «experienced duration» and «remembered duration». A recent meta-analysis by Block and Zakay (1997) underlines the fact that attentional processes are needed to explain prospective judgements, and memory-based processes are needed to explain retrospective

judgements. Thus, in prospective timing, time appears to flow more rapidly if one is engaged in a higher level of behavioural activity. For example, a one-hour interesting conversation with a friend will appear to flow faster than an hour tossing and turning in bed, trying in vain to fall asleep. Yet these effects are reversed in retrospective timing. A time interval filled with interesting events is estimated (recalled) longer than a period devoid of them (Nichelli, 1996). The judgement of the time interval is based on the number of events stocked in memory. If many events are stocked for a given period of time, this period will be recalled as longer than if only a few events had happened. For example, remembering a one-hour period filled with many phone calls, answering e- mails, and meeting your superior will lead to a longer time estimate than remembering a one-hour period attending a dull class.

There are several methods to measure time perception. Bindra and Waksberg (1956) described four methods: (a) verbal estimation, where the subject is asked to estimate verbally in conventional units (seconds, minutes, hour, etc.) an interval given by the experimenter; (b) production, where the subject is instructed to mark the beginning and the end of an interval of a duration stated verbally by the experimenter; (c)

reproduction, where the subject is told to reproduce a duration presented by the

durations is the longest (or the shortest). Several variants of these methods have been developed (see Allan, 1979; Nichelli, 1996; Zakay, 1990). Nevertheless, no method is clearly superior over one another and, furthermore, there appears to be a lack of correlation between the different methods.

Misperceiving sleep onset

Transition from wakefulness to sleep is a complex process. The expression «falling asleep» does not illustrate well what actually occurs. Not only sleep onset is gradual, but frequent fluctuations between wakefulness and sleep are observed during the transition (Badia, Wright and Wauquier, 1994). Although a multitude of subtle changes can be observed in the electroencephalographic (EEG) activity during the transition from wake to sleep (Harsh, 1994; Hasan & Broughton, 1994), some of the main indicators of sleep onset are (a) decline of EEG activity; (b) loss of environment consciousness; (c) decrease of muscle tone; (d) decline of responses to verbal stimuli; and (e) inaccurate time perception (Rechtschaffen, 1994). Some of these indicators, however, are difficult to evaluate without disrupting sleep. Thus, the traditional way to operationally define sleep onset is the use of an arbitrary criterion (the first epoch in stage 2) obtained by EEG recordings (Hauri & Olmstead, 1983). However, for Rechtschaffen (1994), sleep is better defined by its behavioural components (immobility, reduced response at stimulation, stereotypic position, reversibility and sometimes subjective impression). According to this author, it is somewhat utopian to determine a precise point of sleep onset because of its gradual nature, the lack of correlation between sleep onset behaviours, and the

extensive individual variability patterns.

Is one able to perceive his/her own sleep onset? In an attempt to study the detection of brief daytime sleep episodes in good sleepers, Rosenthal et al. (1999) have

obtained the following results: three subjects detected sleep onset after one minute of EEG-defined sleep, seven subjects after five minutes of sleep, ten subjects after ten minutes of sleep, and 14 after 20 minutes of sleep. These authors found the frequency of detecting sleep to be a linear function of prior sleep length.

According to Goldenberg, Benoît and Bastuji (1986), the organisation of sleep in the first sleep cycle would be critical for the estimation of poor or good sleep. The perception of having slept, and having slept well, is dependant upon the length of prior continuous sleep (Sewitch, 1984). Smith and Trinder (2000) have pointed out that arousals during the sleep onset period may lengthen the perceived sleep onset latency. Thus, the more sleep is disrupted, the more unlikely it is for the individual to perceive his/herself asleep.

There is a particular group of persons whose complaints are similar to those suffering from insomnia, but whose PSG data fail to show evidence of sleep disturbances. This condition has been labelled «subjective insomnia» or «sleep state misperception»1. Although this diagnosis is controversial (Reynolds, Kupfer, Buysse, Coble & Yeager, 1991; Trinder, 1988), there is growing interest in studying this condition (Bonnet & Arand, 1997; Dorsey & Bootzin, 1997; McCall & Edinger, 1992; Mendelson, 1993, 1998; Salin-Pascual, Roehrs, Merlotti, Zorick & Roth, 1992; Sugerman, Stem & Walsh, 1985). Salin-Pascual et al. (1992) have reported more stage 1 and 2 sleep and less stage 3 and 4 in patients with sleep state misperception compared with insomniacs with objective findings and good sleepers. This finding is paradoxical because it challenges the

1 Subjective insomnia also often refers to a diagnosis of insomnia done on the unique basis of subjective reports, without access to objective data. Sleep state misperception is diagnosed when the objective findings are not congruent with the subjective reports. It has also been labelled in the literature as «Insomnia without objective findings».

assumption that patients with sleep state misperception have a sleep pattern identical to that of good sleepers. It may be that, although usually reported sleep parameters (sleep latency, total sleep time, wake after sleep onset, etc.) are similar in patients with sleep state misperception and good sleepers, differences could be seen in the composition of their sleep architecture.

In a recent study, ratios of subjective over objective sleep latencies of different groups of people with insomnia complaints have been compiled (Vanable, Aikens, Tadimeti, Caruana-Montaldo & Mendelson, 2000). Ratios varied from 115.0 to 121.5 in people who suffered from objective insomnia, insomnia with depressive disorder,

insomnia secondary to an axis I disorder other than depression, periodic leg movements, and sleep apnea. The largest ratios were observed among patients with sleep state misperception : they produced an average ratio of 131.8. Edinger and Fins (1995) proposed that the magnitude of misperception is a continuum that could be related to a spectrum of different pathologies leading to an insomnia complaint.

Depression, anxiety, and medication : their effects on sleep and time perception

Depression and insomnia interact together to a point where it is often difficult to determine which one is the primary disorder. Besides, the DSM-IV-TR (American Psychiatric Association, 2000) criteria for depression include sleep disturbances, whether in the forni of insomnia or hypersomnia. Research has demonstrated that sleep of

depressed people is different from that of controls (more last-frequency EEG activity at sleep onset, transition to sleep more gradual and variable) (Armitage, Hudson, Fitch & Pechacek, 1994). Bitwise, Friedman, and Yesavage (1993) associated depressed mood to a tendency to underestimate sleep duration. They found a positive correlation between a measure of depression and the extent of the discrepancy between objective and subjective

sleep duration. Furthermore, depressed insomniacs have more difficulty estimating their sleep latency than three other groups of insomniacs (without objective findings,

psychophysiological, medical) (Hauri & Olmstead, 1983).

Insomnia and anxiety are also commonly associated. Bonnet (1990) has reported greater anxiety in insomniacs than in good sleepers. Manipulation of anxiogenic

cognitive intrusions leads to delayed sleep onset and reduced total sleep duration (Gross & Borkovec, 1982; Van Egeren et al., 1983). Morgan, Healey and Healey (1989) indicated that subjective insomniacs aged 65 to 74 showed significantly higher levels of trait and state anxiety and neuroticism when compared to good sleepers. Time perception is also affected by stress, as noted by Moiseeva (1987).

When studying the effects of sedative medication, Mendelson (1993,1995a, 1995b) has reported no effects of flurazepam, zolpidem and triazolam on time perception. Rather, he indicated that these medications affected the perception of either being awake or asleep. Along with this position, Bonnet and Arand (1994) have argued that

decreasing the level of arousal, by sleep deprivation or the administration of sedatives, will decrease the overestimation of sleep latency. Nevertheless, Moiseeva (1987)

indicated that drugs exerting an inhibitory effect on the central nervous system induce an underestimation of time, whereas stimulating drugs and hallucinogenic agents induce an overestimation of duration.

Summary and aim of the present study

It appears that insomniacs are less accurate in estimating sleep parameters than good sleepers, although they display no clear deficit in cognitive performance that could account for this inaccuracy. The greatest discrepancies between objective and subjective measures of sleep parameters are observed among individuals suffering from sleep state

misperception. However, since the definition of sleep onset exclusively by EEG criteria can be questioned, particularly in insomniacs, it is not clear whether the discrepancies result from an inability to perceive time or an inability to perceive sleep state.

The discrepancies between subjective, behavioural, and physiological measures of sleep is a still poorly understood issue that faces clinicians and researchers. Increasing our knowledge about time perception in insomniacs is important for many reasons. First of all, since PSG equipment is quite expensive, most clinicians have to rely on subjective reports of sleep parameters for the evaluation of patients. Second, it has been

hypothesised that the magnitude of sleep latency overestimation could parallel a continuum of pathologies leading to insomnia complaints (Edinger & Fins, 1995; Vanable et al., 2000). Thus, in this perspective, ratios of subjective/objective sleep latency could serve as diagnostic clues. Finally, it has been shown that misperceiving sleep onset, i.e. perceiving oneself awakened, confirmed or not by PSG, leads to medication consumption (Englert & Linden, 1998).

Are insomniacs’ poor judgements of sleep parameters attributable to an

impairment in their time perception ? The present study attempted to shed some light on this issue. The aim was to compare sleep-related and sleep-unrelated time perception in insomniacs and good sleepers. The sleep-related condition focused on sleep onset latency, following Bonnet et al.’s (1982) recommendation. For economical reasons a nap was used for the sleep-related condition. Haynes, Fitzgerald, Shute & Hall (1985) have reported the utility and the validity of naps for the evaluation of sleep onset in insomniacs. Moreover, since McCall and Edinger (1992) reported that insomniacs display sleep difficulties not only at night but also during the day, we can expect the afternoon nap to be an representative equivalent of a night in the sleep laboratory.

The main hypothesis of this experiment was that there would be an interaction between the insomniac or good sleeper status and the sleep-related or the sleep-unrelated condition. What was expected was that insomniacs would overestimate the intervals to a greater extent than good sleepers, but this difference would be observed only during the sleep-related condition.

Method Participants

Participants were recruited by posters placed on the campus of Laval University. Thirty good sleepers (17 women) and 30 insomniacs (18 women) participated in the study. The average age was 23.37 years old (SD = 3.88) for the good sleepers group and 25.64 years old (SD = 8.53) for the insomniacs group. This difference was not

significant.

Selection criteria for the insomniac group were those of the DSM-TV-TR (APA, 2000) for primary insomnia. These criteria were evaluated with the Sleep Impairment Index (SII) and the Insomnia Interview Schedule (IIS) (see Sleep Evaluation section for more details). The participants had to report difficulties to initiate or maintain sleep for at least one month. Sleep disturbance had to cause clinically significant distress or

impairment in social, occupational or other domains of functioning. Although insomnia is primarily a subjective complaint (Borkovec, 1982), more operational criteria were used (Lacks & Morin, 1992). The participants had to report a sleep latency over 30 minutes, or more than 30 minutes of wake after sleep onset, for at least three nights a week. Insomnia did not occur exclusively in the course of another sleep disorder (narcolepsy, sleep apnea, etc.) or another mental disorder (major depression disorder, generalized

anxiety disorder, etc.). Finally, insomnia was not due to the physiological effects of a drag.

The good sleepers reported no difficulty initiating or maintaining sleep and were not taking medications for sleep. Both sleep latency and wake after sleep onset were under or equal to 30 minutes.

Participants of both groups completed the same series of questionnaires. None of them were taking sleep-enhancing medication. All participants received 20$ CAN after they completed the experiment. Referrals to bibliotherapy and sleep clinicians were provided to people suffering from insomnia. If they desired, they also received general treatment guidelines for insomnia treatment, following the recommendations of Hauri (1993) for one-time consultation for insomnia.

Sleep evaluation

Insomnia Interview Schedule (IIS). The IIS (Morin, 1993) is a semi-structured interview developed to evaluate the history of sleep difficulties, to screen out other sleep disorders, and to gauge the relative contribution of psychological, behavioural,

environmental, and medical factors. It also provides guidelines to conduct a functional analysis that includes antecedents, consequences, secondary gains, precipitating and perpetuating factors of insomnia. In the present study, this interview was not used in its original verbal format, but in a written questionnaire format. Also, although the

interview was conceived to be used principally with insomniacs, good sleepers also completed it.

The Sleep Impairment Index (SIT). The SII (Morin, 1993) is a seven-item

quantitative evaluation of sleep difficulties. The participant uses a five-point Likert-type scale (0 = not at all and 4 = extremely) to score the following items: (a) severity of

sleep-onset, sleep maintenance and early awakening problems; (b) satisfaction with current sleep patterns; (c) interference with daily functioning; (d) noticeability of impairment attributed to the sleep problem; and (e) level of distress caused by the sleep problem. The subjective ratings provide valuable information on the person’s perception of his/her sleep problem. Scores range from 0 to 28, with higher scores indicating higher severity. The ISI has adequate internal consistency and is a reliable self-report measure to evaluate perceived sleep difficulties (Bastien, Vallières & Morin, 2001). Two parallel versions (that were not used in the present study) can be completed by a clinician and a significant other (e.g. spouse, friend).

Psychological evaluation

Beck Depression Inventory (BDD. The BDI (Beck, Ward, Mendelson, Mock & Erbaugh, 1961) is a widely used instrument, either in research or in clinical settings (Beck, Steer & Garbín, 1988). It includes 21 items targeting different symptoms of depression. Each item is a four-statement list reflecting different intensities of depressive symptoms. The participant chooses which statement(s) suit(s) better his/her state in the last seven days. Total scores range from 0 to 63: higher scores mean higher severity. Scores between 0 and 10 indicate no significant depressive symptom; between 11 and 16, mild level of depression; between 17 and 20, moderate level; between 21 and 30, severe level; between 31 and 40 very severe level; and over 40, extremely severe level of depression. Psychometric properties of the French version are well-established (Bourque & Beaudette, 1982; Gauthier, Morin, Theriault & Lawson, 1982).

Beck Anxiety Inventory (BAD. The BAI (Beck, Epstein, Brown & Steer, 1988) is a 21-item self-reported questionnaire which evaluates the intensity of anxiety symptoms. The patient rates each cognitive, affective or physiological symptom during the last seven

days on a scale from 0 to 3. Total scores range from 0 to 63, with higher scores indicating higher severity. Scores between 0 and 9 indicate a normal level of anxiety; between 10 and 18, a mild level; between 19 and 29, a moderate level; and 30 and over, a high level. The French version of the BAI has good reliability and validity in non- clinical samples (Freeston, Ladouceur, Thibodeau, Gagnon & Rhéaume, 1994). Procedure

After an initial telephone screening, the participant was invited to a group meeting (three to four participants at a time) with the experimenter where he/she received more information about the study. If he/she accepted to participate, the consent form was read and signed, and the questionnaires (IIS, SIL BDI and BAD were completed.

The procedure used in the sleep-unrelated and sleep-related conditions are fully described in Tables 1 and 2, respectively. The instructions given to the participants are provided verbatim. The order of presentation of sleep-related and sleep-unrelated conditions were counterbalanced.

Insert Tables 1 and 2 about here

Participants were told not to bring any watch for the experiment. Participants who forgot were asked as soon as they arrived to remove their watch for the duration of the experiment. During the sleep-unrelated retrospective interval, participants were in a waiting room. Magazines were available. During the sleep-unrelated prospective intervals, participants were sitting in the bedroom with absolutely nothing to do. If they asked whether they should count in their head to keep track of time, they were told that they could use any strategies they wanted, without having to use one if they did not wish to. During the sleep-related retrospective and prospective intervals, participants were

lying in bed, eyes closed. They were instructed to try to fall asleep. Some participants anticipated that they would not be able to sleep. In these special cases, they were asked to do as if it was a habitual night at home, and the experimenter repeated that they had to try to fall asleep.

Tasks

The tasks used in the experiment involved verbal estimations of time intervals. This kind of task was chosen because it resembles most the situation where an insomniac has to estimate his/her sleep latency for evaluation purposes. The task included

estimations of (a) a retrospective long (12.5 minutes) interval; (b) a prospective short (35 seconds) interval; (c) a prospective medium (3 minutes) interval; and (d) a prospective long (12.5 minutes) interval. Selection of these interval durations were modelled after those used by Moore and collaborators (1980, 1982). Five second-, 35 second-, and 19 minute- intervals were used by these authors. Because the focus of the present study was on the sleep onset period, the five second interval was judged too short, and the 19 minute interval was judged too long. Only the 35 second interval was kept. An interval of 12.5 minutes was chosen because, unlike with 10 or 15 minutes, the participants’ accuracy will not be falsely enhanced by the tendency to report estimates in round

numbers. A medium duration between two round numbers (one and five) was also added to the design (i.e. the three minute interval).

Participants underwent four verbal estimation tasks. This was done twice (in the sleep-related and the sleep-unrelated condition). The first task was a 12.5 minute

retrospective judgement. The participant had to verbally estimate a duration at the end of the interval without knowing that he/she would have to do it. In the sleep-unrelated condition, the interval began at the closing of the experimenter’s office door; in the

sleep-related condition, the interval began at lights off (see Procedure). The end of the intervals was signalled by the experimenter’s questions. The second, third and fourth tasks were 35 second, 3 minute, and 12.5 minute prospective judgements, respectively. The experimenter signalled verbally the interval’s beginning and end. Immediately after, the participant estimated the interval’s duration.

The dependent variable of time perception was the ratio of subjective estimate on the objective duration. A ratio of 1 would mean that the participant has estimated perfectly the duration. A ratio below 1 indicates an underestimation, whereas a ratio greater than 1 indicates an overestimation. The use of ratios enables a comparison of duration judgements across conditions and experiments (Block & Zakay, 1997).

Results

Sleep parameter data, based on a global and retrospective questionnaire (IIS ) completed during the information session, are presented in Table 3. All subjective sleep parameters were different between the groups. Sleep latency [t (57) = -11,145, p < .01], number of awakenings [t (58) = -4,015, p < .01] and wake after sleep onset [t (53) = - 3,849, p < .01] were higher in the insomniac group, whereas total sleep time [t (56) = 5,055, p < .01] and sleep efficiency [t (56) = 4.464, p < .01] were lower. Insomniacs’ mean score was greater than the suggested threshold (Morin, 1993) to declare insomnia of clinical significance. Insomniacs indicated an average of five (SD = 1.58) night per week with sleep disturbances. They reported an average insomnia duration of about seven years (84.85 months; SD = 75.36).

The mean SII scores are reported in Table 4. As expected, there was a significant difference between the groups [t (58) = -11.533, p < .001]. The mean BDI and BAI scores are also reported in Table 4. There were significant between-group differences for the depression measure [t (58) = -4.763, p < .01] and for the anxiety measure [t (58) = -4.863, p < .01]. Referring to the scales proposed by Beck and collaborators (1961,

1988), the good sleepers showed no significant depressive symptom and insomniacs overlapped the «no depressive symptom» and «mild level of depression» categories. Regarding anxiety, once again good sleepers showed a normal level of anxiety whereas insomniacs showed a mild level of anxiety.

Insert Table 4 about here

Every temporal estimation collected during the experiment was transformed into a ratio following Block and Zakay’s (1997) recommendation. Each estimate of a given time interval was divided by the correspondent objective duration. Table 5 presents the means and standard deviations obtained by each group for both conditions and for the four tasks. The data reveals that almost all mean ratios (except for the sleep-unrelated 12.5 minute prospective judgement) were larger in the insomnia group. Moreover, standard deviations were almost always larger in the insomnia group, reflecting a greater variability in the insomniacs’ responses. Hartley’s test for variance homogeneity

revealed that, in the sleep-unrelated condition, insomniacs showed greater variance in the 12.5 minute retrospective judgement |־Fmax (2,29) = 45.203, p < .01] and in the 3 minute prospective judgement [Fmax (2,29) = 3.052, p < .01]. In the sleep-related condition, insomniacs showed greater variance in the 35 second prospective judgement |־Fmax (2,29)

= 10.107, p < .01] and in the 3 minute prospective judgement [Fmax (2,29) = 4.908, p < .01]. The other between-group differences of variances were not significant.

Insert Table 5 about here

Descriptive analyses of the data revealed that most ratios were not normally distributed. A logarithmic transformation was used to normalise these variables. Thus, the subsequent parametric analyses were performed on the logarithm of each ratio.

Thirty-nine participants (18 good sleepers and 21 insomniacs) reported being awake at the end of the sleep-related retrospective long interval and 21 (twelve good sleepers and nine insomniacs) reported being asleep. To detect a possible effect of falling asleep during the nap, a multivariate analysis of variance (ΜΑΝΟΥA) was conducted on the eight log transformed mean ratios. There was no significant effect of being awake or asleep (F < 1) nor was there any significant interaction between the group and being awake or asleep (F < 1) on the log transformed temporal estimates.

Twenty-nine participants (14 good sleepers and 15 insomniacs) began the experiment with the sleep-unrelated condition, whereas 31 (16 good sleepers and 15 insomniacs) began with the sleep-related condition. To detect a possible effect of order of condition, a ΜΑΝΟΥ A was performed on the eight log transformed mean ratios. There was no significant effect of the interaction between the group and the order of conditions (F < 1). However, the main effect of order of conditions was significant [F (8,49) = 3.301, p = .004]. This finding led us to add this variable to our original design in order to separate the effect of order of conditions from the possible effects of group and condition. The means and standard deviations of temporal estimation ratios as a function of the order of condition are presented in Table 6.

Insert Table 6 about here

Regarding our main hypothesis, data were analysed in a split-plot design with two between-group variables (good sleeper or insomniac status and order of conditions) and one within-group variable (sleep-unrelated or sleep-related condition). A repeated measures analysis of variance (ANOVA) was performed on the log transformed ratios of subjective estimates over real durations. This analysis was performed four times, one for each of the four different time estimates (12.5 minutes-retrospective, 35 seconds-

prospective, 3 minutes-prospective, and 12.5 minutes-prospective).

Results of the four repeated measure ANOVAs are summarised in Table 7. For the 12.5 minute retrospective judgement, there was no clear effect of either group (insomniac or good sleeper), condition (sleep-related or sleep-unrelated), order of conditions or any interaction. Nevertheless, the group effect was close to statistical significance [F(l,57) = 3.215, p = .078], with greater ratios obtained by insomniacs. The analysis of the 12.5 minute prospective judgement showed a significant interaction between the group and the condition [F(l,57) = 4.658, p = .035]. Insomniacs’ log transformed mean ratio was greater in the sleep-related condition, while good

sleepers’ greater log transformed mean ratio was in the sleep-unrelated condition. This result confirmed our hypothesis. As can be seen in Figure 1, for good sleepers, time estimates while waiting in a bedroom were longer than when trying to fall asleep. However, for insomniacs, longer estimates are produced in the sleep-related condition.

Insert Figure 1 about here

For the 35 second prospective judgement, the main effect of condition [F(l,57) = 8.659, p = .005] and the effect of the interaction between condition and order of

sleep-unrelated condition had more accurate estimates in the sleep-related condition whereas participants who began with the sleep-related condition had better estimates in the sleep-unrelated condition. In other words, participants were more accurate the second time they did the estimation of the short interval. There was no significant effect of either group, order of conditions, or interaction between group and condition.

For the three minute prospective judgement, the main effect of the order of conditions was significant [F(l,57) = 5.089, p = .028], with greater log transformed mean ratio from participants who did the sleep-unrelated condition first. There was no

significant effects of either group, condition, or interaction between group and condition. Insert Table 7 about here

Discussion

In an attempt to study sleep-related time perception, we compared the

performance of insomniacs and good sleepers on four temporal estimation tasks. The main hypothesis was that insomniacs would overestimate intervals to a greater extent than would good sleepers, but this difference would be observed only in the sleep-related condition. The results showed that insomniacs produced longer estimations than good sleepers for prospective long intervals in the sleep-related condition. For the

retrospective long interval and the short and medium prospective interval, however, the interaction between the group and the condition was not significant. The hypothesis is thus partially confirmed by the results obtained.

Significant differences between conditions were found only for the 35 second interval. For this estimate, the interaction of condition and order of conditions was also significant. Participants in both groups overestimated to a greater extent the short interval when they were waiting in the bedroom with nothing to do than when they were

in bed trying to sleep. This is congruent with the time perception literature: in a

prospective paradigm, time will be perceived to pass more slowly if one does not engage in an interesting activity because most of the attention is focused on the temporal task. However, since time seems to pass more slowly when one is engaged in an unpleasant or dull activity, we would have expected insomniacs to give larger overestimations in the sleep-related condition because of its aversive quality for this particular group. The length of the short interval may have obscured this effect. Moreover, the significant interaction between condition and order of conditions revealed that the first time

participants of both groups estimated the short interval, they considered it longer than the second time. They gave more accurate estimations the second time they did the task. Therefore, there might be a practice effect for this interval. Since there was no

significant interaction between conditions and order of conditions for the medium and long durations, this practice effect seems to be circumscribed to short intervals.

For the medium interval, there was a significant main effect of order of conditions. This result means that participants who began by the sleep-unrelated

condition were more likely to produce longer estimations, in either the sleep-unrelated or the sleep-related condition. Participants who began with the sleep-related condition were more accurate. Thus, when assessing the time perception (particularly short and medium intervals), it is critical to consider the possible practice and order effects. This is rarely evaluated in studies reporting insomniacs’ subjective sleep parameters, which are temporal estimations repeated many times by the same participants.

For the retrospective long (12.5 minutes) interval, there was a trend for

insomniacs to produce longer estimates retrospectively, in either sleep-related or sleep- unrelated condition. This result suggested a generalised impairment of time perception

among insomniacs. However, for the prospective interval of equal duration, the interaction between condition and group was significant, whereas there was no main effect of group. In the prospective paradigm, insomniacs were more accurate than good sleepers in the sleep-unrelated condition, and the opposite was found for the sleep-related condition, i.e. insomniacs were less accurate. Therefore, these results suggest an

impairment of time perception among insomniacs present only in sleep-related settings. For good sleepers, waiting in a room with nothing to do can be very dull, and may be perceived as lasting longer than trying to fall asleep, the latter being usually a pleasant and relaxing situation. However, for insomniacs, it does not seem more pleasant to sleep than to wait. Indeed, longer estimates are produced in the sleep-related condition,

suggesting that, for this group, lying in a bed trying to sleep would be a more aversive situation than waiting in a room with nothing to do.

In summary, these results indicated that insomniacs were less accurate than good sleepers when estimating long intervals. Furthermore, this impairment seemed to be generalised when they estimated time retrospectively, but only sleep-related when estimating durations prospectively. In other words, when they did not know that there would be a temporal interval to be estimated, insomniacs overestimated time intervals more than good sleepers. However, when they were aware that they would have to provide an estimate of a given duration, insomniacs overestimated time intervals only in sleep-related situations.

Several methodological limitations may have prevented us from observing clearer differences. First of all, the exclusive use of a retrospective, subjective and global

questionnaire to form the groups is the principal flaw of the present study. In absence of PSG data, completion of sleep diaries on a one- or two-week basis would have provided a

more precise distinction between insomniacs and good sleepers, and enhanced the validity of our groups. Second, it is questionable whether the use of a nap in this

particular experiment is equivalent to a regular night of sleep. Daytime time perception might not be the same as night-time time perception. The factors contributing to

insomnia and time perception impairments at night might not have been present in the experimental setting during the day. Besides, nine insomniacs reported being asleep during the nap, only three less than in the good sleepers group. Although preliminary analyses showed that perceiving oneself awake or asleep did not influence the temporal estimates, the finding that almost as many insomniacs as good sleepers fell asleep is somewhat intriguing.

According to the stimulus control theory, however, this result is not surprising. This theory stipulates that insomnia is the result of maladaptive conditioning between environmental stimuli and sleep-incompatible behaviours (Morin, 1993). For example, a person could be constantly worrying (sleep-incompatible behaviour) while lying in bed (environmental stimulus) trying to fall asleep. The environmental stimuli thus gradually lose their association with sleep, leading to insomnia. This is also why some insomniacs sometimes sleep better when away from home, the sleep-incompatible stimuli being absent in new sleep settings. In our study, the experimental settings being different from the participant’s usual bedroom environment, it is plausible that the conditioning of environmental cues with hyperarousal was absent. We may not have created a situation similar to a typical night of poor sleep, hence the absence of the typical time perception errors.

The alternative hypothesis of a sleep state misperception, instead of a time perception deficit, to explain the discrepancies between objective and subjective