Circadian variation and soccer performance: Implications for training and match-play during Ramadan

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Circadian variation and soccer performance:

Implications for training and match-play during Ramadan

B. Drust

, Q. Ahmed

& R. Roky

a

Research Institute for Sport and Exercise Sciences, Liverpool John Moores University , Liverpool , UK

b

Attending Sleep Disorders Medicine, Division of Pulmonary Disease and Critical Care Medicine, Department of Medicine, Winthrop University Hospital , Mineola New York , United States

c

Laboratory of Physiology and Molecular Genetics, Neurobiology Unit, Faculty of Sciences , University of Hassan II Ain Chock , Casablanca , Morocco

Published online: 09 Jul 2012.

To cite this article: B. Drust , Q. Ahmed & R. Roky (2012) Circadian variation and soccer performance: Implications for training and match-play during Ramadan, Journal of Sports Sciences, 30:sup1, S43-S52, DOI: 10.1080/02640414.2012.703784 To link to this article: http://dx.doi.org/10.1080/02640414.2012.703784

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Circadian variation and soccer performance: Implications for training and match-play during Ramadan

B. DRUST

, Q. AHMED

, & R. ROKY

1

Research Institute for Sport and Exercise Sciences, Liverpool John Moores University, Liverpool, UK,

Attending Sleep Disorders Medicine, Division of Pulmonary Disease and Critical Care Medicine, Department of Medicine, Winthrop University Hospital, Mineola New York, United States, and

Laboratory of Physiology and Molecular Genetics, Neurobiology Unit, Faculty of Sciences, University of Hassan II Ain Chock, Casablanca, Morocco

(Accepted 13 June 2012)

Abstract

Ramadan results in a number of behavioural alterations in individuals when compared to their normal habits outside of this holy month. These changes in behaviour could impact upon the effectiveness of the activity of an elite athlete who has high daily activity levels and energy expenditures. Understanding the true impact of Ramadan on human physiology will also require an awareness of the key aspects of circadian rhythms. This article will present theoretical background content on circadian rhythms along with data on the potential influence of circadian variation on soccer performance. It will also attempt to provide an insight into the problems of partial sleep deprivation and travel for the elite player. The contents will suggest that there is a basis for the within-day variation in physiological and psychological function to impact soccer performance if games are played early in the day or very late at night. As competitive fixtures are uncommon at these times these influences may be more relevant to the timing and organisation of training sessions. It is also likely that a lack of sleep and excessive travel will provide conditions that are not conducive to optimal performance. This would indicate that teams should think carefully about their preparation strategies for important tournaments and games.

Keywords: Circadian rhythms, performance, soccer

Introduction

The holy period of Ramadan requires Muslims to change their normal behaviours with respect to eating, drinking and sleeping. Changes in this combination of factors are likely to affect daytime activities and performance in the mental, physical and social domains (Waterhouse, 2010). The partly endogenous diurnal variation that exists in an individual’s physiological function is also thought to influence performance in a number of everyday physical and mental tasks (Reilly, Atkinson, &

Waterhouse, 1997). Understanding the potential impact of Ramadan on the performance of elite soccer players may require an awareness of the key features of the body clock and the impact that circadian rhythms can have on the accomplishment of tasks. This article attempts to provide an overview of circadian rhythms and soccer. Such information will provide a useful theoretical and practical frame- work for the understanding of the influence of

Ramadan on the players during competition and training.

Circadian rhythms

Rhythms and periodicity are robust predictive homeostatic processes in both humans and other living organisms. Rhythms can be observed in both biological and behavioural parameters with these rhythms exhibiting different periods. The periodicity of important rhythms can vary from months (infra- dian) to a matter of hours or minutes (ultradian). A circadian rhythm is one which recurs around the solar day exhibiting a length of around 24 hours. A good example of a circadian rhythm is the rhythm in core temperature. Figure 1 illustrates the pattern of the daily variation in core temperature and includes an illustration of both the peak value (acrophase) that occurs around 17:00 to 18:00 and the minimum noted around 6:00. In addition to the circadian rhythm observed in body temperature are rhythms in

Correspondence: Barry Drust, Research Institute for Sport and Exercise Sciences, Liverpool John Moores University, Tom Reilly Building, Liverpool, L3 2ET, United Kingdom. E-mail: b.drust@ljmu.ac.uk

Journal of Sports Sciences, 2012; 30(S1): S43–S52

ISSN 0264-0414 print/ISSN 1466-447X onlineÓ2012 Taylor & Francis http://dx.doi.org/10.1080/02640414.2012.703784

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the sleep-wake cycle, hormone release, alertness, psychomotor performance, short term memory and mood (Borbe´ly, 2009; Drust, Waterhouse, Atkinson, Edwards, & Reilly, 2005; Johnson et al, 1992;

Morris, Aeschbach, & Scheer, 2012; Murray, Alan,

& Trinder, 2002).

The biological clocks within the body ensure the organisation of the sleep-wake cycle and prepare the metabolism and cardiovascular system for increased activity (Kalsbeek et al., 2011). This shows that the circadian system is partly responsible for the optimisation of the daily rhythms in order to permit adaptations to the environmental changes that occur during the day. These circadian rhythms are controlled by the endogenous clocks which are located in the suprachiasmatic nuclei (Stephan &

Zucker, 1972). This master circadian pacemaker generates circadian rhythmicity and synchronises numerous subsidiary local clocks in other regions of the brain and peripheral tissues. This suggests that the circadian system in mammals is ‘‘multioscilla- tory’’ in nature with a number of other anatomi- cally distinct oscillators existing within the body.

The location and properties of these potential circadian pacemakers that lie outside of the suprachiastmatic nucleus region remain unknown.

These endogenous clocks run with a period longer than 24 hours as the time in isolation studies is around 25 hours and about 24.2 hours in more carefully controlled lighting conditions (Czeisler et al., 1999).

At the molecular level, the circadian clock mechanism originates from interconnected loops in gene expression that operate in a cell-autonomous and self-sustained fashion. Approximately 10 ‘‘clock genes’’, including PER1/2/3, CRY 1/2, BMAL1, CLOCK, and casein kinase 1delta/epsilon (CK1d/e) interact to generate oscillations of specific transcripts and proteins (Hirayama & Sassone-Corsi, 2005) though the negative PER/CRY feedback loop is

commonly seen as the primary generator of the circadian rhythm. These genes feed-back on their own expression via the interaction with CLOCK/

BMAL1 (Bode, Shahmoradi, Rossner, & Oster, 2011).

Even if the circadian endogenous period is very close to 24 hours, it will gradually diverge from the environment if it is not synchronised by periodic signals. Biological clocks are entrained to the environment by zeitgebers. Several periodic cues are capable of entraining the oscillator. Photic stimulus is the most relevant cue although several other stimuli are also important for the training of the circadian rhythms. Among these other potential synchroniser agents food availability, social contact and exercise are the most frequently scientifically studied (Aschoff et al., 1971, Atkinson, Edwards, Reilly, & Waterhouse, 2007). It has also been suggested that it is the combination of these factors that are the most influential zeitgebers in humans (e.g. a mixture of bright light and social factors).

Research indicates that there are a number of other individual characteristics that can influence the circadian variation observed in humans. Several studies have demonstrated that there are inter- individual differences in sleep time and in body temperature related to the morning–evening pre- ference of individuals. These differences are re- flected by morning types choosing to sleep on average around 2 h earlier than evening types (Mongrain, Carrier, & Dumont, 2006). Gender differences in the timing of human circadian rhythms have also been demonstrated in several studies. On average, women tend to wake up earlier than men and exhibit a greater preference for morning activities (Randler, 2011). Women may also have a significantly higher melatonin amplitude and lower temperature amplitude than men (Cain et al., 2010). Women also seem to have a lower tolerance to shift work (Saksvik, Bjorvatn , Hetland, Sandal, & Pallesen, 2011). These differences may be related to a significantly shorter intrinsic period for rhythms in women than in men (Duffy et al., 2011). These shorter average intrinsic circadian periods observed in women may have implications for understanding the gender differences in habitual sleep duration and insomnia and sleepiness pre- valence in humans.

Human performance, especially during exercise is underpinned by the physiological and psychological function of the individual. It is therefore logical to assume that diurnal changes in the mental and physical responses observed in circadian rhythm research could therefore influence the performance of athletes in all sports. Research evidence would suggest that many measures of human performance follow closely the curve in body temperature

Figure 1. Circadian rhythm in rectal temperature (based on figures in Reilly, Atkinson & Waterhouse, 1997).

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(Reilly, Atkinson, & Waterhouse, 2000). These would include components of motor performance that are important in athletic events such as strength, reaction time, jumping and endurance (Drust et al., 2005). Another major biological rhythm that is of major importance to athletes is the sleep-wake cycle.

The sleep-wake cycle results in sharp contrasts in arousal with peak values occurring around mid-day at the time that the concentrations of adrenaline are at their highest. Such changes in arousal may influence important mental aspects of performance.

The influence of circadian rhythms and performance in soccer

The logical starting point for examining the potential influence that circadian variation could have on the performance of the soccer player is an understanding of the demands of the sport. Performance in soccer is a consequence of a number of inter-dependent factors (see Figure 2) that include the technical and tactical abilities of the player, their psychological make-up and their physiological capabilities. Each of these individual attributes can then be further sub- divided into a number of important sub-compo- nents. For example, the physiological demands of soccer require players to possess high levels of aerobic fitness, an ability to sprint (in repeated bouts as well as one-off efforts), a capability for force generation (muscle strength) and a need to be flexible (Svensson & Drust, 2005). The relative contribution of all of these factors to performance is not necessarily consistent across different playing populations (e.g. young players, recreational male players, elite female players) or within a specific sub-

set of players. This is probably a consequence of modifications in the required performance charac- teristics for each player(s). Such changes are a direct result of the inherent features of the game such as speed of play and the specific requirements asso- ciated with different team formations and playing positions. It would seem therefore that any successful performance in the sport of soccer is underpinned by a diverse range of related individual physiological and psychological attributes. This would suggest that there may be multiple ways in which the circadian variation in human function could influence the ability of players to perform in games.

The scientific investigation of the influence of circadian variation on soccer performance requires the application of suitable experimental models.

Such models should include both appropriate theoretical and methodological considerations. Two approaches are commonly applied when researching the impact of variables on performance within the field of science and soccer. The most ecologically valid of these methods is to use the actual perfor- mance of players in games. For example, teams may be asked to play games at different times of the day (e.g. 06:00 and 18:00) while instrumented to record physiological/psychological information. This data could then be compared to highlight any potential circadian effects on the relevant evaluated perfor- mance outcomes. Such approaches, though undeni- ably effective intuitively are, however, limited in a large number of ways. These include the inability to take appropriate measurements during match-play due to rules and regulations, the difficulty of obtaining permission to use players in competitive games and the general lack of experimental control within ‘‘real world’’ situations. This lack of control can result in substantial ‘‘noise’’ in the data collected. These ideas are supported by the large variability that is associated with a range of perfor- mance variables that are commonly used as indica- tors of a player’s match activity in motion-analysis studies. For example, the coefficient of variation (CV) for high speed running and sprinting in matches was around 16% and 31% respectively in the large sample of players analysed by Gregson, Drust, Atkinson, and Salvo (2010) using a commer- cially available computer automated system. These data would indicate that such variables are difficult to implement as a model of performance unless a large sample of games is available for a given individual as they are too variable. This requirement may be impractical for the vast majority of researchers in the area of chronobiology.

The other methodological approach that is com- monly used is to decompose the factors that under- pin match performance into discrete physiological/

psychological capabilities and then devise

Figure 2. Determinants of soccer performance with specific reference to the physiological demands.

Circadian variation and soccer S45

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appropriate assessment protocols to evaluate each specific parameter(s) in isolation. The underlying assumption of such approaches being that the analysis of a relevant discrete component(s) will provide an idea of the changes in capability in an element(s) of performance relevant to soccer as a function of that given experimental manipulation.

This in turn may provide useful data on the potential to perform in an actual soccer performance. Such tests clearly need to be specific to the sport as unsuitable protocols (e.g., mode of exercise) will limit the relevance of the data for the competitive situation. While this approach clearly enables the experimental control required to make sound scien- tific judgments it is probably limited by being overly simplistic in a deterministic sense. Despite this potential limitation this approach has been the most commonly used within the scientific literature especially with respect to studies of relevance to circadian variation and sport performance (Drust et al., 2005).

Despite its popularity as a sport there have been surprisingly few scientific studies that have provided us with specific information on the circadian varia- tion in soccer performance or demonstrated rele- vance to the sport by using high-level soccer players as participants. As a consequence of the restrictions in the specific literature that is available, data from more general studies on sport performance that have relevance to soccer will be included in this article.

The main determinants of soccer performance as described in the previous paragraphs will form the framework for developing a case for the potential influence of circadian variation on performance. The focus will predominantly be on the physiological characteristics relevant to the game as these have been the most widely researched. More comprehen- sive reviews on the influence of circadian variation on sport performance are available for the interested reader who requires a broader perspective of the area than can be provided here (e.g. Drust et al., 2005;

Reilly et al., 2000).

Evidence relating to aerobic fitness

The aerobic energy system is an important energy pathway in soccer. Changes in a player’s potential to maximally or fractionally utilise the oxygen transport system may have major implications for his/her ability to perform the specific physical activity required to fulfill his/her technical/tactical role within the team. In both longitudinal (Reilly & Brooks, 1982) and cross-sectional (Faria & Drummond, 1982; Reilly & Brooks, 1990) studies, maximal oxygen consumption ( V O _

) is a stable function, independent of the time of day of measurement. This would suggest that maximal aerobic performance

would not be compromised if games were played across different times of the day. The ability to tolerate fixed-intensity work-rates close to V O _

are, however, significantly higher in the afternoon compared to the morning (Hill, Borden, Darnaby, Hendricks, & Hill, 1992, Reilly & Baxter, 1983).

Such prolonged periods of activity are unlikely to occur in soccer due to the frequent breaks in play though these findings may have relevance for particularly intense periods of the match when the energy demands will be similarly high.

Other relevant circadian rhythms may include those detected in parameters that have relevance to the physiological responses to aerobic exercise.

Horne and Petit (1984) were unable to detect rhythmicity in the V O _

responses to sub-maximal exercise though other investigators (Giacomoni, Bernard, Gavarry, Altare, & Falgairette, 1999; Hill, 1996) have demonstrated rhythms during sub-max- imal exercise that peak from 04:00 to 14:00 hours with a range of 13% (Reilly & Brooks, 1982). The V O _

kinetics (Faisal, Beavers, & Hughson, 2010) and the time required for V O _

to reach steady state (Reilly, 1982) does not seem to vary with time of day suggesting stability in the rest to exercise transition irrespective of time. Other variables such as blood pressure (Deschenes et al., 1998) and heart rate also seem to exhibit circadian rhythmicity (Callard et al., 2001; Reilly, 1982). These observations may have more relevance for the understanding of physiological data that is collected during matches and training than for the physical performance of players.

Evidence relating to anaerobic performance Circadian rhythms have been identified in some laboratory measures of anaerobic performance.

Reilly and Down (1992) observed significant circa- dian rhythmicity in the standing broad jump, with an acrophase of 17:45 hours and an amplitude of 3.4%

of the 24-hour mean value, when individual differ- ences in performances were accounted for. Similar rhythm characteristics have also been found for anaerobic power output in a stair run (Reilly &

Down, 1992) and vertical jumping performance (Taylor, Cronin, Gill, Chapman, & Sheppard, 2011; Teo, McGuigan, & Newton, 2011), but not for sprint times (Bernard, Giacomoni, Gavarry, Seymat, & Falgairette, 1998). This may indicate that there is potential for some technical actions such as heading to be affected in matches that could be played at certain times of the day. The potential impact of any such changes on the outcome of the match would, however, probably be very small due to the relative infrequency of such actions in a player’s overall activity pattern.

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Activities that last a longer duration (410 s) that include a significant contribution from the anaerobic energy system (e.g. Wingate test) also demonstrate increased peak and mean power outputs in the evening compared to at 03:00 hours (Deschodt &

Arsac, 2004; Souissi et al., 2010). Short duration maximal efforts that are repeated with short recov- eries (repeated sprint activities) probably have more relevance to soccer than prolonged supra-maximal efforts of 30 s in duration. Racinais, Perrey, Denis, and Bishop (2010) demonstrated that initial sprint performance (sprints 1, 2 and 3) in a 10 sprint protocol was faster between 17:00–19:00 than 08:00–10:00. Decrements in power across the repeated sprint protocol did not show a similar circadian variation once differences in the initial power output were normalised between morning and afternoon. This would indicate that the fatigability of the muscle during intense periods of match-play may be more resistant to circadian changes than one-off bouts of anaerobic activity.

Circadian variations in specific muscle function Both strength and flexibility are important to the soccer player. Flexibility is crucial for the perfor- mance of static and dynamic soccer-specific actions that occur at the end point of an individual’s range of motion. Strength in individual muscle groups seems to be related to the performance of a range of specific technical skills such as kicking, jumping and tackling.

The existence of circadian variation in such indica- tors of specific muscle function would therefore seem to have the potential to influence soccer performance by affecting a player’s ability to complete important match actions. Changes in flexibility, and to a lesser extent muscle strength, may also have the potential to increase the likelihood of muscle injury.

The available evidence on the influence of circadian variation on both flexibility and muscle strength is pretty conclusive. Both muscle strength and flexibility exhibit a circadian rhythm with performance consistently peaking in the early eve- ning. The circadian variation in muscle strength is, however, partly dependent upon the muscle group tested (Strutton, Catley, & Davey, 2003) and the mode of muscle contraction (Giacomoni et al., 1999). The exact mechanism of these changes in strength is as yet unresolved though both peripheral and central factors would seem to be implicated in the changes in force (Martin, Carpentier, Guissard, Van Hoeke, & Duchateau, 1999). Circadian varia- tion in flexibility was noted in lumbar flexion and extension, gleno-humeral lateral rotation and whole- body forward flexion (Gifford, 1987, Manire et al., 2010) but not in spinal hyperextension, lateral movement of the spine and ankle plantar and dorsi-

flexion (Edwards & Atkinson, 1998). There are, however, large inter-individual differences of be- tween 12:00 and 24:00 hours in the peak-times of flexibility (Gifford, 1987). These discrepancies are probably a consequence of methodological differ- ences between investigations and/or insensitivities in measurement techniques rather than evidence for the discrete modulation of diurnal variation in flexibility within specific muscle groups.

Daily variation in attributes that have relevance to the tactical and technical performance of players

A player’s tactical and technical performance is a consequence of a large number of co-ordinated psychological and physiological processes. This creates the potential for such skills to be influenced by circadian changes in any number of factors that relate to sensory motor, psychomotor, perceptual and cognitive function. The relative insensitivity of the outcome measures used in the majority of the available research, and the limitations in the experi- mental designs that can implemented, may lead to the influence that circadian variation has on actual performance in a complex task such as soccer match- play being underestimated. This would suggest that this particular area represents a vibrant research field for future scientists and practitioners.

Circadian rhythms are present in several elements of sensory motor, psychomotor, perceptual and cognitive function (Winget, DeRoshia, & Holley, 1985) though few of these investigations provide us with information that is directly translatable to soccer performance. Simple reaction time (either to audi- tory or visual stimuli) is fastest in the early evening at the same time as the maximum in body temperature (Reilly et al., 1997). An inverse relationship between the speed and the accuracy with which a simple repetitive test is performed is, however, often observed making accuracy worse in the early evening (Atkinson & Spiers, 1998). Similar tasks which demand fine motor control (e.g. hand steadiness and the ability to balance) are performed better in the morning, since arousal levels will be lower than the diurnal peak and closer to the optimum level for performance (Colquhoun, 1972). Circadian varia- tion has also been demonstrated in soccer-specific motor skills such as dribbling, chipping and ball juggling (Reilly, Fairhurst, Edwards, & Waterhouse, 2004a; Reilly, Farrelly, Edwards, & Waterhouse, 2004b). Complex aspects of performance such as mental arithmetic and short-term memory also peak in the early hours of the morning rather than the evening (Conroy & O’Brien, 1974) though the rhythm is influenced by the load characteristics of the task (Winget et al., 1985).

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Training

Preparing players for competition is an important part of the activities performed by coaches, managers and support staff within soccer. The structure and content of training will help players prepare for the next competition by delivering appropriate physiolo- gical loads that allow players to maintain fitness while also facilitating recovery from the load of previous matches. Longer term periods of training can help ensure that specific aspects of a player’s character- istics can be developed to help ensure higher performance levels. These improvements may be the difference between players attaining a starting place on the team and non-selection. Chronobiolo- gical factors may play a role in optimising the training response. Carrying out specific types of training sessions at specific times of the day (if practically possible) may influence the body’s adaptive response by either (a) creating a psychological/physiological state that permits an individual to train harder or (b) providing a magnified signal for adaptation com- pared to other times of the day.

Few, if any, experimental studies exist that have tested the influence of changing the time of day on the adaptive response to a training stimulus, espe- cially in relation to soccer. The available evidence, some of which is discussed in the previous sections, would seem to suggest that circadian factors could influence the outcomes of training sessions. For example, the circadian variations in both hormone concentrations and performance may influence the adaptations associated with a strength training programme (Hayes, Bickerstaff, & Baker, 2010).

Individual perceptions of effort are also highly influenced by the time of day (Teo et al., 2011).

This may change a player’s willingness to exercise at a high intensity within a training session. Alterations in sensory motor, psychomotor, perceptual and cognitive function within a day may also suggest that complex technical and tactical coaching sessions are better performed in the morning when arousal is lower. It would therefore seem potentially useful to consider the timing of specific types of training sessions, especially when more than one session is completed in any given day.

Disruptions of the circadian rhythms

Circadian rhythm disorders arise when there is dysfunction of the circadian endogenous clock or its entrainment pathways or when there is a misalignment between the timing of the endogenous circadian rhythms and the external environment.

Such circadian rhythm disorders can be chronic in which an individual’s biological circadian rhythm is out of phase with sleep and wake periods necessary

for conforming with conventional environmental patterns (Rohers & Roth, 1994) or more transient.

Two common transient and extrinsic disorders are shift work and jet lag (American Academy of Sleep Medicine, 2005; Sack et al., 2007). In these two disorders, there is a misalignment between the timing of the endogenous circadian rhythms and the external environment. In contrast to the intrinsic circadian rhythm disorders, circadian disruptions in relation to jet lag and shift work are more frequent.

The disassociation between the internal rhythms and the external environment in the general popula- tion following periods of travel and disrupted sleep (associated with shift work) influences a range of physiological and psychological performance char- acteristics. It would therefore seem logical to assume that such effects are also relevant to the soccer playing population. The remaining sections of this paper will briefly examine the impact of travel and sleep disruptions on soccer players.

Travel

The modern soccer player is required to travel significant distances as a consequence of the compe- titive fixture calendar (see Table I). This travel may be confined to a team’s country of residence or include trips within the same continent or across different continents. Time away from home travel- ling for the elite player is very common as individuals in successful teams could play up to three games a week, sometimes all away from the home stadium.

Travel for these elite teams is frequently organised to be as comfortable as possible with all seating in first or business class. Such measures, while reducing the stresses and strains of the journey, do not completely remove the difficulties that are always inherent in moving a relatively large number of individuals any distances.

Travel, especially by air, can lead to travel fatigue and/or jet lag. These two conditions are not the same thing as they are differentiated by the type of travel that precedes any symptoms and the nature and duration of the signs of complaint once at destination.

Both travel fatigue and jet lag have the potential to influence soccer performance though the available data on athletic populations is greater for jet lag. It is currently unclear which may pose the biggest challenge to soccer teams though the movement across multiple time-zones for major club and international competitions provides evidence of the need to consider jet lag. Jet lag refers to the feelings of disorientation, light headedness, impatience, lack of energy, loss of appetite, difficulty in sleeping, poor concentration and co-ordination and general dis- comfort that follow travel across time zones (Reilly et al., 1997). Such feelings may linger for several days

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after arrival, though there are large inter-individual differences in the extent that players may experience such symptoms and the time course such feelings may persist. This change in mental and physiological state clearly has potential to disrupt performance and lead to a failure of teams to progress within competitions that may generate significant prestige and/or prize money. For example, jet lag may reduce those fine motor skills needed for technical actions, negatively influence concentration at key parts of the game and also reduce the ability and the willingness of the player to fulfill demanding activity profiles. It should also be noted that these issues will also affect managerial, coaching and medical staff and poten- tially their ability to make key decisions related to both the tactical performance of the team and organisational strategies related to the visit.

The symptoms of jet lag are, in part, a conse- quence of the failure of the body’s circadian rhythms to adjust to the time at destination. For example, a rapid transition across the time zones that separate the UK and Australia will lead to a 10 hour change in time. This may mean that the newly arrived individual may be forced to be awake when they would normally be sleeping if at home. In time the new environment, the time of sunrise and darkness, and activity and social contact will force the circadian cycles to adjust in time. This re-synchronisation of the important circadian rhythms (such as those of core temperature and arousal) will in time lead to a loss of the symptoms of jet lag and a return to the performance levels usually observed at home.

The severity of jet lag is influenced by various factors. Crossing a large number of time zones is more difficult for the majority of people to deal with than shorter flights. This may mean that the travel that is usually typical of European teams in Union of European Football Associations (UEFA) competi- tions is more easily tolerated than flights that are more typical within large continents such as Asia and the Americas. The direction of travel is also an important factor in the severity of jet lag. Research evidence would suggest that is far easier to cope with flying in a westward than an eastward direction (Waterhouse,

Reilly, Atkinson, & Edwards, 2007). This is because the body rhythms can extend their natural time period during westward travel as opposed to a forcible shortening of the cycle when travelling eastward.

Maximal time zone shifts of around 12 hours seem however to result in little difference whether travel is either westward or eastward in direction.

A number of strategies can be employed in an attempt to facilitate the re-synchronisation of an individual’s circadian rhythms and hence minimise the impact that travel may have on their performance.

An important initial consideration is time of depar- ture and time of arrival as inappropriate travel strategies on either the outward or the return leg may prevent appropriate levels of re-synchronisation.

Exercise can be an especially important factor in such strategies as there is evidence to suggest that activity can speed up the adaptation to a new time zone (Waterhouse et al., 2007). Suitable travel arrange- ments can facilitate players to immediately adopt the phase characteristics of the new environment. For example, the travel schedule could be arranged so that the players are able to complete a light training session in the evening upon arrival. This will not only help anchor the body temperature rhythm to its new time zone but also delay the onset of sleep to a time that is more suitable. Alternatively training should be avoided if the arrival is scheduled for the morning when activity could promote the anchoring of the body clock to the departed time zone (e.g. after a long-haul flight to the east).

Irrespective of the timing of the initial training session the activities included should be adapted for a short time period (2–3 days) following arrival.

Soccer skills that require fine co-ordination are likely to be disrupted for a few days in the new environ- ment as a consequence of the de-synchronisation of relevant circadian rhythms. These changes may lead to accidents or injuries as players seek to regain their technical and physical competencies. Reducing the intensity of the initial sessions may also help prevent unsuitable physiological loads on players as they adjust to both the new climate and time zones. This may mean that it is beneficial to incorporate friendly

Table I. Travel and training match/schedule of a Premier League Team during a pre-season tour to Asia.

Day 1 Day 2 Day 3 Day 4 Day 5 Day 6 Day 7 Day 8 Day 9

Morning Flight to

far east (Total flight time¼18 hours)

Arrive UK (Total flight time¼19 hours) Afternoon Flight to

far east

Training Training Training Training

Evening Flight to far east

Training Training MatchFlight to new training venue (Total flight time¼4 hours)

Training Training MatchFlight to new training venue (Total flight time¼2 hours)

Training Flight to UK

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games during the first week if the travel has been undertaken to play important competitive games in which positive results are required. The timing of sleep is also crucial in any strategy to minimise the effects of jet lag as napping at an inappropriate time (a time that the player should have been asleep at home) for prolonged periods will only make sub- sequent sleep more difficult and prolong the adjust- ment of the biological clock.

Sleep

Recently the impact of sleep on athletic performance has begun to be considered. Indeed sleep is now considered as a new frontier in performance en- hancement though the scientific investigations to support this assumption is only just beginning. Even though detailed information on the science of sleep and exercise is not available, athletes and their trainers have long recognised that getting adequate

‘good’ sleep and ‘proper rest’’ can aide performance as well as improve recovery from competition related myalgias, muscular injuries or mental burn out.

Accumulated sleep loss, often known as sleep debt, has negative impacts on cognitive function, mood and daytime sleepiness, reaction time, learning and memory tasks (Carskadon & Dement, 1981;

Dinges et al., 1997; Van Dongen, Maislin, Mulling- ton, & Dinges, 2003). Some studies have also demonstrated a negative relationship between sleep deficit and physical performances including weight lifting and cardiorespiratory functioning (Mougin et al., 1991, Reilly & Piercy, 1994). Taken together this data would suggest that adequate sleep is necessary for optimal performance.

Adequate sleep appears also to have a major influence on the ability of an athlete to exert effort (Engle-Friedman, Palencar, & Riela, 2010). Eigh- teen competitive ice-skaters were studied via self- report questionnaires that included information on major sleep time, timing of training, and skating preferences including complexity and perceived difficulty of manoeuvres. Those who subjectively experienced more night time awakenings selected less challenging manometers during their practices with total sleep time correlating negatively with the perceived difficulty of selected manoeuvres. Sleep loss would therefore seem an important indicator of not only how well a task may be performed but also how likely an athlete is to carry out specific complicated actions. This may influence the ability of players to perform in games and training.

Determining the sleep patterns of athletes, espe- cially young players, is one barrier to influencing the amount of sleep that is obtained in these populations.

The sleep needs of the player frequently go unnoticed as the majority of physicians do not

seem to regularly enquire about sleep routines (Owens, 2001). Information is also difficult to obtain from significant others as parents also do not know of their child’s usual sleep latency or night-time awakenings (Fricke-Oerkermann et al., 2007). This would suggest that there is a great opportunity to educate those who work with players to influence the approaches to sleep observed in both individuals and teams.

Recently investigators raised the question as to whether increased amounts of sleep could enhance athletic performance. Investigators at Stanford an- swered this question by studying 11 healthy students on the Stanford University Basketball team (Mah, Mah, Kezirian, & Dement, 2011). Participants were asked to voluntarily extend their total sleep time recorded in self-report surveys for a 5–7 week period.

Pre-test total sleep opportunities were recorded in self-report sleep surveys prior to intervention. Essen- tially the participants were asked to forgo the sacrifice of sleep for other activities with a minimum goal of a 10 hour minimum sleep opportunity for their major sleep period. Participants were then assessed using recognised measures of basketball performance in- cluding a timed sprint and basketball shooting accuracy. Participants were also assessed for vigilance with standard measures such as assessments of their vigilance measures, alertness and ability to respond to stimuli. Following the intervention period participants were found to have enhanced basketball performance according to all measured variables. Total sleep times increased by 110.9 + 79.7 min and participants were shown to sprint faster and have greater shooting accuracy than their baseline performance. Alertness also improved as measured by psychomotor vigilance task (PVT) testing and mood, sleepiness and fatigue also improved. These findings lead the investigators to conclude that optimising sleep need, or reaching sleep satiation was likely to have a positive impact on measured objective performance in sport. As basket- ball represents a similar sport to soccer, in a number of ways, these findings could have relevance for the sport in question.

Exercise itself may have significant impact on sleep including notable changes in measured sleep archi- tecture. Exercise timed sufficiently in advance of bedtime can reduce sleep latency (the time taken to fall asleep). Exercise too close to bedtime will have the opposite effect because of the rise in core body temperature which must then reduce before sleep.

Polysomonographic measures of sleep in vigorous exercisers show shortened latency times to sleep onset, increased sleep efficiency, increased slow wave sleep percentages (also known as Stage 3 sleep) and reduced light sleep (Stage 1 and 2) percentages (Brand, Beck, Gerber, Hatzinger, & Holsboer- Traschler, 2010).

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These findings suggest a beneficial impact of exercise on sleep. The mechanisms for these changes are not clear but likely multifactorial.

Conclusions

This article has attempted to provide some back- ground information on the influence of circadian rhythms on soccer performance. This content provides a useful framework to facilitate the under- standing of the influence of Ramadan on human exercise performance. The available research would seem to suggest that there is an influence of the time of day on physiological and psychological factors that are relevant to soccer performance. This literature base is, however, limited in the number of studies that have used test protocols that have high ecological validity to actual soccer performance and/or used high level soccer player as participants.

This makes firm conclusions regarding the impact of circadian variation on actual soccer performance difficult to make at this current time. The compo- nent circadian rhythms that may have relevance to key aspects of soccer do seem to peak at different times of the day. This would theoretically make it difficult to optimally time either competitions or training. Such scheduling may, however, be imprac- tical due to the organisational constraints on elite teams and players in the modern game.

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