CIRCADIAN RHYTHMS IN SPORTS PERFORMANCE—
B. Drust, J. Waterhouse, G. Atkinson, B. Edwards, and T. Reilly
Research Institute for Sport and Exercise Sciences, Liverpool John Moores University,
Henry Cotton Campus, Liverpool, UK
We discuss current knowledge on the description, impact, and underlying causes of
circadian rhythmicity in sports performance. We argue that there is a wealth of
information from both applied and experimental work, which, when considered
together, suggests that sports performance is affected by time of day in normal
entrained conditions and that the variation has at least some input from endogenous
mechanisms. Nevertheless, precise information on the relative importance of
endogenous and exogenous factors is lacking. No single study can answer both the
applied and basic research questions that are relevant to this topic, but an
appropriate mixture of real-world research on rhythm disturbances and tightly
controlled experiments involving forced desynchronization protocols is needed.
Important issues, which should be considered by any chronobiologist interested in
sports and exercise, include how representative the study sample and the selected
performance tests are, test-retest reliability, as well as overall design of the experiment.
Keywords Athletes, Performance Analogues, Jet Lag, Endogenous and Exogenous
Chronobiologists could be considered to have two main preoccupa-
tions. First, they are obviously interested in the “hands of the clock”;
biological rhythm characteristics (e.g., amplitude or acrophase) are
described, and the impact of these characteristics on real world situations
(e.g., transmeridian travel) is appraised. Second, chronobiologists are
interested in the “mechanisms of the clock”; the origins of a particular
biological rhythm are elucidated through various experiments, in which
competing exogenous sources of rhythmicity are systematically removed
or accounted for.
Address correspondence to G. Atkinson, Research Institute for Sport and Exercise Sciences,
Liverpool John Moores University, Henry Cotton Campus, 15–20 Webster Street, Liverpool, L3
2ET, UK. E-mail: G.Atkinson@livjm.ac.uk
Chronobiology International, 22(1): 21–44, (2005)
Copyright # 2005 Taylor & Francis, Inc.
ISSN 0742-0528 print/1525-6073 online
The above two preoccupations are obviously not mutually exclusive.
The elucidation of the mechanism of a particular biological rhythm may
ultimately lead to the development of treatments for rhythm disorders
in real-world situations. For example, melatonin is thought to have a
role in the control of circadian rhythms and consequently has been
hypothesized to reduce symptoms of jet lag in athletes (Atkinson et al.,
2003). Conversely, the behavior of a biological rhythm in a real-world situ-
ation may offer some insight into the underlying mechanisms of that
rhythm. Observations, such as the persistence of circadian rhythms
during sleep deprivation or the desynchronization of rhythms during jet
lag, support the presence of endogenous control of circadian rhythms
(Minors and Waterhouse, 1981). Therefore, it is important to consider
the mechanisms and impact of circadian rhythms in tandem.
It is difﬁcult to describe circadian rhythms in variables relevant to sports
and exercise, while at the same time provide information on their underlying
mechanisms. When an athlete is administered a test of physical performance,
there are obvious physiological responses to the exercise involved, which in
turn might “mask” any underlying mechanisms. This masking might be
acute or long-lasting, dependingon the variable measured, and is a particular
problem for research on the zeitgeber qualities of exercise (Edwards et al.,
2002). Not surprisingly therefore, most researchers (e.g., Atkinson and
Reilly, 1996) have, in the past, concentrated on describing the various
rhythms and discussing the impact on real sports competitions, rather than
elucidating the root causes of circadian rhythmicity in sports performance.
Recently, Youngstedt and O’Connor (1999) criticized research on cir-
cadian rhythms in athletic performance. These authors maintained that
there is no evidence that circadian rhythmicity in real sports competitions
is explained by an endogenous component. They also suggested that the
lack of evidence for an endogenous rhythm in physical performance
means that sports performance is not affected by either time of day nor cir-
cadian rhythm disturbances such as jet lag, when various exogenous
factors are taken into account. Interestingly, the assertions made by
Youngstedt and O’Connor (1999) did not restrict these authors from the
anomalous position of providing advice to travelling athletes on the basis
of chronobiological principles (see O’Connor et al., 2004).
Youngstedt and O’Connor’s (1999) review should encourage a deeper
analysis of the available data and direct the priorities for future research.
Nevertheless, we maintain that there is information concerning both the
mechanisms and “hands” of circadian rhythms in athletic performance,
which was not considered by Youngstedt and O’Connor (1999). We aim,
in this review, to: (1) appraise the impact of circadian rhythms in sports
performance on real athletes in real sports competitions, and (2) discuss
the evidence for and against circadian rhythms in sports performance
being mediated endogenously.
B. Drust et al.22
THE IMPACT OF CIRCADIAN RHYTHMS ON SPORTS
Notwithstanding any discussions about the relative importance of
endogenous and exogenous mechanisms to circadian rhythms in sports
performance, it could be argued that if sports performance does vary
with time of day in normal everyday conditions, then this has direct
impact on the athlete. Performance that occurs outside of the “peak
window” over a 24 h period may be potentially less than optimal (Winget
et al., 1985) with the within-day variation being greater than that required
to differentiate between successful and unsuccessful performers (Hopkins
et al., 1999). This impact makes an understanding of the circadian
variation in sports performance an important practical consideration for
both athletes and coaches in competition (Cappaert, 1999) and might
have important implications for both the short- and long-term success of
an athlete or team. There is also an impact on athletic training, where
the motivational climate of competitive stress is absent, and the training
stimulus is highly dependent on the athlete’s input of effort.
Important issues at this applied end of the research continuum
(Atkinson and Nevill, 2001) are the size of the sample, how representative
the research sample is of an athletic population, how well the selected
performance tests in the research represent real sports performance,
and how well the research design controls for intervening variables, such
as learning effects.
Indirect Evidence for Circadian Variation in Sports Performance
Indirect evidence for the existence of circadian rhythms in sports per-
formance comes from examination of the times of day at which athletes
perform best (or worst) in actual sports events. The element, which is maxi-
mized in this type of “ex post facto” examination, is obviously external
validity of sample and performances (Atkinson and Nevill, 2001). Previous
evaluations of world record breaking performances in sports events seem
to indicate a circadian variation (Atkinson et al., 1999) with world records
broken by athletes competing in the early evening, the time of day at which
body temperature is highest.
We have stressed in a number of publications (Atkinson and Reilly,
1996; Reilly et al., 2000) that such ex post facto research should, of course,
be interpreted with caution, since there is a bias for scheduling ﬁnals of
track and ﬁeld competitions in the afternoon or evening due to extraneous
inﬂuences such as the demands of television. The lack of control over
environmental inﬂuences on performance is also a major problem con-
cerning evidence from ﬁeld-based studies. For instance, environmental
temperature may also be more favorable to record-breaking performances
Circadian Rhythms in Sports Performance 23
in the evening, especially in the summer (Youngstedt and O’Connor,
1999). Circadian ﬂuctuations in meteorological conditions such as wind
speed and direction may also affect performances in cycling or ﬁeld
sports involving high velocities of projectiles (e.g., discus, javelin, hammer).
The bias in event-scheduling for the early evening can, in some sports,
be controlled. In time trials in competitive cycling the frequency of races is
more evenly distributed throughout the daylight hours. The performances
of young competitors in 16-km races are better when held in the afternoon
and evening compared to those scheduled in the morning (Atkinson,
1994). When the frequency of trials is standardized at different times of
the day in simulated competitions, weight-throwers also perform better
in the evening than the morning (Conroy and O’Brien, 1974). Tighter
control of environmental conditions can be achieved in swimming,
where water conditions are held constant throughout the day. Data on
swimming performance indicate improved performances for both 100-m
and 400-m swims in the afternoon or early evening (Baxter and Reilly,
1983). Similar observations have also been made by others (Rodahl
et al., 1976; Arnett, 2002).
The lack of full control over extraneous inﬂuences on performance is
the major problem concerning the evidence outlined above. The need for
more conclusive proof of the existence of circadian variation in perform-
ance has led to researchers carrying out controlled laboratory-based inves-
tigations. These studies are, however, associated with both theoretical and
General Experimental Design
“Performance,” in the context of a sporting action, has a broad
meaning (Atkinson, 2002). Successful performance can be dependent on
different combinations of ﬁne motor skills, gross motor performance,
and cognitive function. Researchers in laboratory-based investigations
have tended to isolate separate components of performance and describe
the circadian characteristics of each, thereby inferring the ecological
validity of the data to the real event. Many of the components measured
(e.g., time trials) are directly relevant to sporting events. Variables based
on the physiological responses to exercise can be considered to be less
relevant to real performance in this respect. While Youngstedt and
O’Connor (1999) criticized the representative nature of some variables
selected in chronobiological studies into sports performance, they cited
the results of a study, which involved the physiological and subjective
B. Drust et al.24
responses to swimming (rather than swimming times) as evidence against
the notion that jet lag hinders sports performance.
It is also essential that the methodological techniques utilized in such
laboratory-based studies approximate the required accuracy needed to
discriminate between successful and unsuccessful performances within
the competitive environment. This difference may be as small as 1%
(Hopkins et al., 1999). The detection of such differences requires investi-
gators to display substantial scientiﬁc rigor in experimental design and
collect data with minimal measurement error. Sample size can also
impact on the conclusions derived.
One point worth noting is that, in discussing the impact of measure-
ment error and sample size on circadian rhythm research, too much
error in the measurement of performance and/or an insufﬁcient sample
size is a type II issue in research. A type II error occurs when a chrono-
biologist has, after completing an experiment, concluded that circadian
rhythmicity is absent for a particular variable when, in reality (in the popu-
lation), circadian rhythmicity is apparent (Atkinson and Nevill, 1998;
Atkinson and Reilly, 1999). Too much measurement error and/or a
small sample size means that the “true” circadian rhythm could not be
detected. It follows, therefore, that if a circadian rhythm is actually
detected in an experiment (in terms of both statistical and practical signiﬁ-
cance), then this ﬁnding cannot be criticized on the basis of too much error
or too small a sample size. The issue in this situation is a type I error. The
inﬂuence of sample size on type I error rates is controlled for naturally in
the mathematics underpinning statistical inference. If sample size is low, so
are the degrees of freedom in the analysis, making it more difﬁcult for a
given test statistic to be deemed statistically signiﬁcant or a conﬁdence
interval to have sufﬁcient precision. A type I error occurs due to biased
sampling of participants or incorrect choice of a statistical test, such as
employing multiple t-tests on time series data rather than a repeated
measures general linear model (Atkinson, 2001).
There are additional problems when a study on circadian rhythms in
performance is attempted. For example, it is difﬁcult to administer a large
number of consecutive performance tests to humans without eliciting a
serial fatigue effect. Consequently, most chronobiological investigations
into human performance have employed some sort of transverse design
(several subjects studied at a few times of day). Each subject performs
the ﬁrst test session at a different time of day. Conventionally, six or
twelve subjects are examined to form what has been termed a “cyclic
Latin square” design (Folkard and Monk, 1980). This protocol removes
the inﬂuence of any learning/fatigue effects that could occur. The
optimal design would allow at least eight hours between each of the test
sessions, so that the measurements are taken over several days. The
advantages of this are that subjects could sleep normally between
Circadian Rhythms in Sports Performance 25
22:00–06:00 h and 02:00–10:00 h and that the performance variables are
examined over two cycles of the circadian rhythm. Alternatively, each of
the test sessions could be administered on different days, although the rig-
orous control procedures would need to be adhered to for 6 days.
Alongside the above general considerations for laboratory investi-
gations, there may be speciﬁc issues with each type of performance variable
(e.g., cognitive, psychomotor or physical tasks), which are examined.
Rhythms in Psychomotor Performance and Motor Skills
Circadian rhythms are present in several elements of sensory motor,
psychomotor, perceptual, and cognitive function (Winget et al., 1985).
Simple reaction time (either to auditory or visual stimuli) is fastest in the
early evening at the same time as the maximum in body tempera-
ture (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 and 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). Complex aspects of performance such as mental arithmetic and
short-term memory also peak in the early hours of the morning rather
than in the evening (Conroy and O’Brien, 1974), though the rhythm is
inﬂuenced by the load characteristics of the task (Winget et al., 1985).
The accuracy and consistency of both badminton (Edwards et al., 2004)
and tennis serves (Atkinson and Spiers, 1998) seem to vary with time of day
with higher accuracy observed at 14:00 than 18:00 h (Figure 1). These
variations appear to be related more to changes in “fatigue” and “basal
arousal” than temperature (Edwards et al., 2004). Circadian rhythms are
also observed for soccer-speciﬁc performance tests such as chipping, drib-
bling, and juggling (Reilly et al., 2004a, b).
The fact that components of a certain sports activity might be inﬂu-
enced by time of day in different ways is interesting from a mechanisms
perspective. For example, it would be difﬁcult to ascribe differences in
the peak circadian times of tennis serve velocity and accuracy to external
inﬂuences or inadequacies in experimental design, since these obser-
vations were made with the same participants in the same conditions.
Nevertheless, some performance components (e.g., ﬂexibility) are more
difﬁcult to separate in terms of endogenous or exogenous mechanisms.
Gifford (1987) noted circadian variation in lumbar ﬂexion and exten-
sion, gleno-humeral lateral rotation, and whole-body forward ﬂexion.
B. Drust et al.26
Variations are not, however, noted in spinal hyperextension, lateral move-
ment of the spine and ankle plantar, and dorsi-ﬂexion in other investi-
gations (Edwards and Atkinson, 1998). Circadian variation in stiffness
(resistance to motion) of the knee joint is similar to that of body tempera-
ture with the lowest levels of stiffness being recorded in the early evening
(Wright et al., 1969). There can, however, be large interindividual differ-
ences of between 12:00 and 24:00 h in the peak-times of ﬂexibility (Gifford,
1987). This variation does not seem to be related to the amount of prior
activity as the completion of a 30 min submaximal warm up does not
remove within-day differences in whole body ﬂexibility (Edwards and
Muscle strength, independent of the muscle group measured or speed
of contraction, consistently peaks in the early evening (Reilly et al., 2000).
The rhythm in isometric grip strength peaks between 14:00 and 19:00 h
with an amplitude of about 6% of the 24 h mean (Reilly et al., 1997).
Other muscle groups, e.g., quadriceps (Callard et al., 2000) and adductor
pollicis (Martin et al., 1999), exhibit similar rhythm characteristics, though
FIGURE 1 Diurnal variation in tennis serving velocity and accuracy (drawn from data in Atkinson and
Spiers, 1998). (
) Signiﬁcant difference compared to other two times of day. (#) Signiﬁcant difference
from 09:00 h.
Circadian Rhythms in Sports Performance 27
these rhythms are variable and are dependent on the muscle group tested
(Gauthier et al., 1996; Strutton et al., 2003) and the mode of muscle con-
traction (Giacomoni et al., 2004). Elbow ﬂexion strength varies with time of
day, peaking in the early evening (Frievalds et al., 1983; Gauthier et al.,
1996, 1997). Back strength is also higher in the evening than the
morning. The rhythm has an amplitude of around 6% of the 24 h mean
(Coldwells et al., 1993). When the isometric strength of the knee extensors
is measured consecutively during the waking hours of the solar day, two
diurnal peaks are evident; one at the end of the morning and another in
the late afternoon/early evening (Reilly, 1990; Reilly et al., 1997). The
exact mechanism of these changes in strength are as yet unresolved with
both peripheral and central factors, as well as a subharmonic in the circa-
dian rhythm, being implicated (Callard et al., 2000; Martin et al., 1999).
Both concentric and eccentric strength have been measured at differ-
ent times of the solar day using isokinetic dynamometry (Atkinson et al.,
1995; Cabri et al., 1988; Ishee and Titlow, 1986). A time of day effect in
these variables has been noted at 1.05, 3.14, 4.19, and 5.24 rad.s
peak values occurring in the early evening (Wyse et al., 1994; Gauthier
et al., 2001; Souissi et al., 2002). Other researchers suggest that the
within-day variation in isokinetic performance variables (e.g., peak
torque, average power, maximal work) are only observed at faster vel-
ocities (3.14 rad
) (Deschenes et al., 1998). This may be a result of
speed-speciﬁc circadian variations in muscle strength that occur as a
result of ﬁber type recruitment patterns. These circadian differences do
not, however, extend to the shape of the relationship between torque
and angular velocity that remains unchanged throughout the day (Gau-
thier et al., 2001). The characteristics of the rhythms observed in
dynamic strength are similar to those observed for isometric contractions,
indicating that the features of the circadian rhythms of the musculoskeletal
system can be studied using different types of muscle action (Gauthier
et al., 2001). The observed rhythms in strength do not seem to be seen
as clearly in females (Phillips, 1994) unless superimposed electrical
twitches are applied to the muscle (Bambaeichi et al., 2004). This ﬁnding
may be related to gender differences in muscle mass affecting the ampli-
tude of the rhythm or to central command playing a greater role in
females compared to males.
The presence of circadian rhythms in short-term (1 min or less) per-
formance is controversial and may be dependent on the type of exercise
performed and the muscle group tested (Bernard et al., 1998). Circadian
rhythms have been identiﬁed in some laboratory measures of anaerobic
power and conventional tests of short-term dynamic activity. Reilly and
B. Drust et al.28
Down (1986) observed signiﬁcant circadian rhythmicity in length of jump,
with an acrophase of 17:45 h and an amplitude of 3.4 % of the 24 h mean
value, when individual differences in performances were accounted for.
Similar rhythm characteristics have also been found for anaerobic power
output in a stair run (Reilly and Down, 1992) and vertical jumping per-
formance (Atkinson, 1994; Bernard et al., 1998), but not for sprint times
(Bernard et al., 1998).
Short-term performance can also be evaluated using short duration
(10–30 sec) maximal ergometer tests. Hill and Smith (1991) measured
anaerobic power and capacity with a modiﬁed version of the Wingate
test at 03:00, 09:00, 15:00, and 21:00 h. Peak and mean power outputs
in the evening were found to be higher than at 03:00 h. A circadian
rhythm in maximal peak and mean power has also been observed by
Souissi et al., (2002, 2003), Melhim (1993), and Deschodt and Arsac
(2004). Other studies using similar procedures for both leg and arm ergo-
metry (Down et al., 1985; Reilly and Down, 1986) have not conﬁrmed that
performance in the Wingate test depends on time of day. Such differences
may be the result of differences in experimental methodologies and the
level of sensitivity in the tests employed.
The classical Wingate test was modiﬁed by Reilly and Marshall (1991)
for performance on a “swim bench.” Even though the fourteen competi-
tive swimmers, who volunteered for the study, trained routinely in the
early morning, mean and peak power peaked in the evening (18:00 h)
with amplitudes of 11 and 14%, respectively.
The results of studies that have examined the effects of time of day on
ﬁxed-intensity work-rates close to maximal oxygen uptake (V
more conclusive than those that have employed supra-maximal exercise
such as the Wingate test. Such work-rates would be relevant to athletic
events lasting 3–6 min, since this is the typical time an athlete can
endure a work-rate equivalent to V
. Hill et al. (1992) reported that
total work performed in high-intensity constant work-rate exercise on a
cycle ergometer was signiﬁcantly higher in the afternoon compared to
the morning. These results agree with the ﬁndings of Reilly and Baxter
(1983) who reported longer work-times (and consequently higher peak
lactate production) when set bouts of high intensity exercise was per-
formed at 22:00 h compared to 06:30 h.
Physiological Responses to Prolonged Exercise
Although the physiological responses to exercise are not directly
indicative of real athletic performance outcomes, the results of studies
on circadian rhythms in these measurements are useful, especially from
a mechanisms perspective. The fact that a physiological response to a
preset intensity and duration of submaximal exercise varies with time of
Circadian Rhythms in Sports Performance 29
day is suggestive of some endogenous control of these responses, since
factors like motivation to perform maximally are not relevant.
Circadian variation in some cardiovascular parameters such as cardiac
output during exercise has yet to be identiﬁed (Reilly et al., 2000). Other
variables such as blood pressure (Deschenes et al., 1998) and heart rate
seem to exhibit circadian rhythmicity (Reilly, 1982; Callard et al., 2001),
although there is difﬁculty in measuring such parameters with the
required accuracy under exercise conditions. Cohen and Muehl (1977)
measured heart rate at rest, during exercise on a rowing ergometer, and
in the recovery period of this exercise at seven times of the solar day
with the lowest heart rates occurring between 04:00 and 08:00 h. This tem-
poral pattern was evident both during and after exercise.
The intensity of exercise may be an important determining factor in
the observed circadian oscillation (Reilly and Garrett, 1995; Giacomoni
et al., 1999). Wahlberg and A
strand (1973) exercised 20 male subjects at
03:00 and 15:00 h and at both submaximal and maximal exercise loads.
Heart rates during exercise were consistently lower at night (3–
) irrespective of work rate. Other studies using an incremen-
tal exercise task have demonstrated the heart rate response just prior to
exhaustion, when exercise intensity is maximal, does not vary with time
of day (Cohen, 1980). Such discrepancies may be related to a reduction
in the range of circadian variability with increasing levels of exercise
(Winget et al., 1985) or a failure to detect the circadian rhythm as
the ceiling of physiological capability is reached during the exercise test
(Atkinson and Reilly, 1996).
The circadian variation in the metabolic response to submaximal exer-
cise is not as conclusive as that of cardiovascular responses, such as heart
rate. Horne and Pettit (1984) were unable to detect rhythmicity in the
responses to submaximal exercise in untrained athletes. Other inves-
tigations (Hill, 1996; Giacomoni et al., 1999) demonstrated rhythms
during submaximal exercise that peak from 14:00 to 17:00 h with a
range of 13% (Reilly and Brooks, 1982). The time required for V
to reach steady state (expressed as the ﬁfth minute value) was not more
than 2 min at light exercise intensities and did not vary with time of day
(Reilly, 1982). No circadian variations were found for expired carbon
dioxide or the time for the respiratory exchange ratio to stabilize during
exercise. The amplitude of the rhythm in minute ventilation is ampliﬁed
during light or moderate exercise. Reilly and Brooks (1982) found that
the ventilatory response to exercise displayed rhythmicity that was
phased similar to the resting rhythm but 20–40% higher in terms of ampli-
tude. The lack of rhythmicity in metabolic responses to exercise is unequi-
vocal when measured at maximal exercise intensities though Deschenes
et al. (1998) have noted a trend for higher values at later points in the
day. In both longitudinal (Reilly and Brooks, 1982) and cross-sectional
B. Drust et al.30
(Faria and Drummond, 1982; Reilly and Brooks, 1990) studies, it has been
found that V
is a stable function, independent of the time of day of
measurement. A critical methodological practice is that subjects not satisfy-
ing the criteria that V
is actually attained (at any time of day) should
be recalled to undergo another test.
The lactate threshold, deﬁned as the point at which blood lactate
increases exponentially with exercise intensity (Yeh et al., 1983), is com-
monly used as an indicator of aerobic ﬁtness. It determines the upper
limit at which aerobic exercise can be sustained. Forsyth and Reilly
(2004) have recently examined the impact of circadian variation on
the lactate threshold performed on a rowing ergometer. Reliable
methods of evaluating lactated threshold, such as the D
(Cheng et al., 1992) seem to exhibit circadian variation, which are reﬂected
in changes in physiological parameters such as heart rate and V
ﬁndings are invaluable to athletic populations that use such physiological
responses to inform training intensities and indicate that lactate threshold
tests should be conducted at the same time of day at which the athlete
usually trains or competes for accurate prediction of training intensities.
Most of the above research work has concentrated on the physiological
responses to exercise in absolute terms rather than in terms of a change in
response from the preexercise level. Such an exploration is important
again from a mechanisms perspective, since one would be able to separate
resting and exercise responses fully. For example, Aldemir et al. (2000)
found that the rise, and not just the absolute level, of body temperature
during exercise depended on time of day, with a greater rate of change
in temperature being observed in the morning.
Cycling activity is frequently used in laboratory conditions to simulate
competitive performances. Time trial performance (16.1 km) is improved
at 17:30 compared to 07:30 h under laboratory conditions (Atkinson et al.,
2005; Figure 2). This may be related to a diurnal variation in pacing
strategies with prolonged bouts of cycling that seem to be related to the cir-
cadian variation observed in core temperature (Reilly and Garrett, 1995;
Figure 3). Such changes in performance may also be accompanied by
alterations in an individual’s preferred pedal rate and pedal velocity as
such variables also vary with the time of day (Moussay et al., 2002,
2003). These changes may reﬂect alterations in recruitment pattern to
beneﬁt coordination and to minimize neuromuscular fatigue as a result
of alterations in ankle mobilization rather than changes in muscle torque.
The severity of exercise may be assessed by asking the individual to
rate it subjectively on a numerical scale. The subjective reactions to
exercise may also be dependent on the time of measurement. Faria and
Circadian Rhythms in Sports Performance 31
Drummond (1982) employed a crossover treatment and reverse-sequence
design to examine the effects of time of day on ratings of perceived exer-
tion (RPE) during graded exercise on a treadmill. The results revealed that
there was a dissociation between RPE and heart rate, which depended on
time of day. The RPE was higher during exercise carried out in the early
hours of the morning (02:00–04:00 h) than in the evening (20:00–
FIGURE 2 Diurnal variation in 16.1-km time trial performance completed with and without a
prolonged pre-exercise warm-up (drawn from data in Atkinson et al., 2005). (
) – times signiﬁcantly
faster in the afternoon compared to the respective morning trail. (#) – time in the morning trial
after a warm-up still signiﬁcantly slower than the afternoon trail performed without a warm-up.
FIGURE 3 Diurnal variation in self-chosen work rate (bars) and body temperature (line points) during
prolonged (60 min) exercise (adapted from Reilly and Garrett, 1995). Open bars – afternoon work-
rate, ﬁlled bars – morning work-rate, open symbols – afternoon temperature, ﬁlled symbols – morning
B. Drust et al.32
22:00 h). Reilly (1990) criticized this study since work rates were set relative
to elicited heart rates of 130, 150, and 170 beats
. Submaximal heart
rate at a set work rate is lowest at night (see above). Therefore, the higher
subjective ratings reported at this time may have been due to higher exer-
cise intensities and not any circadian variation in RPE per se. Studies that
have exercised subjects at levels expressed relative to V
heart rate have found a circadian variation in RPE at intensities corre-
sponding to lactate threshold (Martin et al., 2001) and maximal exercise
(Reilly et al., 2000). However, low-intensity exercise when performed
many times within a solar day may mediate a transient increase in RPE
in the early afternoon (Reilly et al., 1997).
How Sound Are the Experiments?
Youngstedt and O’Connor (1999) argued that circadian variation in
performance could be due to variations in dietary habits. Many researchers
have measured performance in the morning test session after an overnight
fast, but allowed the subjects to eat up to 3 h prior to the afternoon and
early evening test sessions. Such differences in dietary controls may
mediate circadian changes in glycogen status. Nevertheless, circadian
rhythms have been documented for performance components that are
not inﬂuenced by glycogen status, such as reaction time and anaerobic
power (Atkinson and Reilly, 1996). There are also interesting differences
in pacing strategy during prolonged exercise, which seem to indicate
some independence from dietary status prior to exercise (Reilly and
Youngstedt and O’Connor (1999) thought it possible that the circadian
variation in some performance measures is due to the fact that the
morning test session follows a period of prolonged immobility during
sleep. Evidence against this hypothesis includes the fact that there is
often a “turnaround” between 22:00 and 02:00 h when performance
decreases, even though subjects have been awake for many hours. In
addition and as discussed in detail in the next section, performance
rhythms are evident in sleep deprivation studies (constant and prolonged
level of activity). Finally, recent work from our laboratory indicates that
the diurnal changes in cycling time trial performance are evident even
after a vigorous warm-up has been undertaken prior to the morning test
(Atkinson et al., 2005; Figure 2).
Circadian rhythms in performance could be explained by general
differences in the time of day at which individuals schedule their activity
and sleep (Youngstedt and O’Connor, 1999). The evidence from research
does not agree with this hypothesis. Several researchers (e.g., Boivin et al.,
1992) have shown that performance rhythms are only slightly different
between extremes of “chronotype” (morning and evening individuals).
Circadian Rhythms in Sports Performance 33
The participants involved in the study by Atkinson et al. (2005) were high
in morningness, but selected higher power outputs (without any feedback
on what these power outputs were) during cycling in the afternoon. This
observation does not support the hypothesis forwarded by Youngstedt
and O’Connor (1999) that performance rhythms could be prone to
“expectancy” inﬂuences. In addition, Piercy and Lack (1988) found only
a slight difference in the phasing of circadian rhythms following six week
periods of athletic training in the morning compared to the same
amount of training in the evening.
Youngstedt and O’Connor (1999) postulated that performance is
higher in the afternoon because the participants are more rested from a
previous day’s exercise. However, this factor has been controlled in the
research design of many studies in that physical activity prior to the tests
has been discouraged and the order of test times has been counterbalanced
(one subject’s ﬁrst test session would be in the morning while another
subject would be tested ﬁrst in the evening) (Atkinson et al., 2005).
The results and characteristics of many of the above studies suggest
that there is, in part, an endogenous component to human performance.
Nevertheless, there are other chronobiological experiments and proto-
cols, which can be performed to quantify fully the relative inﬂuence of
endogenous and exogenous factors. Chronobiological theory and proto-
cols enable the causes of observed rhythms to be ascertained, though
this has rarely been applied to sports performance. A knowledge of
these causes would provide a full rationale for giving advice to the athlete.
A CHRONOBIOLOGICAL PERSPECTIVE ON THE MECHANISMS
OF CIRCADIAN RHYTHMS IN SPORTS PERFORMANCE
There are two methods by which the separation of endogenous and
exogenous components can be made experimentally. The ﬁrst of these
requires the direct (exogenous) effects of the environment and sleep-wake
cycle to be reduced as much as possible. A protocol to do this is called a
“constant routine” (Mills et al., 1978; Czeisler et al., 1985). When this protocol
is applied to the circadian rhythm of core temperature (which will be affected
by the sleep-activity cycle), it is performed as follows. The subject is required:
to stay awake and sedentary (or, preferably, lying down and relaxed) for
at least 24 h in an environment of constant temperature, humidity, and
lighting; to engage in similar activities throughout, generally reading or
listening to music, and to take, identical meals at regularly-spaced intervals.
When this protocol is undertaken, it is observed (Figure 4) that the rhythm
of core temperature does not disappear, even though its amplitude
becomes decreased. Three deductions can be made from this result:
B. Drust et al.34
1. The component of the temperature rhythm that remains must arise
from within the body. This is the “endogenous” component and it is
attributed to the “body clock.”
2. The effect of the environment and the sleep-wake cycle can be inferred
from the fact that the two curves differ; this difference is the exogenous
component of the original rhythm.
3. In subjects living normally (as is the case in Figure 4), these two com-
ponents are in phase, both raising core temperature in the daytime
and lowering it at night.
The second method for separating the exogenous and endogenous
components of a circadian rhythm is “forced desynchronization” (Kleitman
and Kleitman, 1953; Boivin et al., 1992). This approach is based on the
observation that the body clock is not able to adjust to an imposed lifestyle
whose period differs substantially from 24 h—that is, is outside the range of
entrainment. Therefore, if subjects live on “days” of 27 h in length (with 9 h
of sleep and 18 h of activity each “day”), their exogenous component is
equal to this period; however, the endogenous component of the circadian
rhythm cannot follow this imposition but rather retains a period close to
24 h—its intrinsic period, or tau. With this protocol, 8 27 h “days”
approximately equal 9 tau, and this length of time is called a “beat
cycle.” One result of this protocol is that the endogenous component of
the rhythm moves continually out of phase with the sleep-wake cycle
FIGURE 4 Mean circadian changes in core (rectal) temperature measured hourly in 8 subjects: living
normally and sleeping from 24:00 to 08:00 h (solid line); and then woken at 04:00 h and spending the
next 24 h on a “constant routine” (dashed line). (Based on Minors and Waterhouse, 1981.)
Circadian Rhythms in Sports Performance 35
and then back into phase. If a variable (for example, core temperature) is
measured regularly throughout a beat cycle, then it can be averaged in one
of two ways. First, if the results are averaged using tau as the reference
time, then any phase of this average rhythm is mixed with all phases of
the imposed (27 h) sleep-wake cycle. That is, provided that the activity
during the sleep-wake cycle is similar day-by-day, any effects due to it
will be averaged out, and the rhythm observed will represent the endogen-
ous component of the measured rhythm. Second, if the temperatures
are averaged using 27 h as the reference time, then any phase of this
average rhythm is mixed with all phases of the endogenous (period ¼ tau)
cycle. That is, any effects due to the body clock will be averaged out, and
the rhythm observed will represent the exogenous component of the
Application of Chronobiological Principles to Athletic
It is clear from the above that the demonstration of circadian rhythmi-
city in physical performance when subjects are living normally does not
enable the relative importance of the exogenous and endogenous com-
ponents of the rhythm to be completely determined. Direct investigations
of the endogenous and exogenous components of physical activity require
measurements to be made during constant routines or forced desynchro-
nization protocols. However, no such studies seem to have been per-
formed. The main reason for this with regard to the constant routine
protocol is that performing the tests would exert a direct effect upon vari-
ables, such as core temperature, so negating the aim of the protocol. (It will
be recalled that this protocol requires the subjects to be resting through-
out.) Such a criticism is less cogent when applied to the forced desynchro-
nization protocol, however, and there would seem to be value in
perfoming such studies.
Even so, there is one study (Reilly and Walsh, 1981) in which the heart
rate and percentage of time active was measured indoors in four soccer
players during a sponsored exhibition, in which self-chosen activity was
continuous for about 86 h. During this time, the environment and activity
regimen were fairly constant, the players being required to be active for
55 min each hour. Therefore, the study can be regarded as one using a
modiﬁed version of a constant routine. The results showed a trend in
both variables to decrease with time but, superimposed upon this, there
were transient falls in heart rate and activity in the middle of each
nighttime, and transient peaks in the middle of each daytime. In
another study (Davenne and Lagarde, 1995), elite cyclists cycled for 24 h
at a constant rate (estimated to be 45% of their maximal capacity as
measured in the daytime) in a gymnasium that was maintained at a
B. Drust et al.36
constant temperature. Subjects were allowed to stop for massages, and it
was observed that the frequency and length of these increased between
02:00–08:00 h. These studies suggest, respectively, that self-paced activity
is less in the middle of the night than in the middle of the day and that, if
the amount of activity is constant, then it is harder to sustain in the middle
of the night. While the results cannot be attributed to the environment
(which remained constant), the extent to which the rhythms observed
reﬂect changes in the ability to exercise and/or the motivation to do so
remains unclear, but the results indicate that some endogenous com-
ponent is present.
The results also illustrate a general parallelism between rhythms of
physical performance and core temperature. (Core temperature was
measured in the study of Davenne and Lagarde, 1995, and can reason-
ably be assumed to have been normally phased in the study of Reilly
and Walsh, 1981.) This parallelism has been found in many studies that
have been performed under normal conditions, and a causal link
between the two has often been assumed, since activities of the muscles
and nerves, the cardiovascular and respiratory systems, and metabolism
are all promoted by a rise of temperature. However, such an interpret-
ation does lead to the paradoxical suggestion that, since the resting temp-
erature is lowest in the middle of the night, the greatest amount of exercise
could be performed at this time, since core temperature could rise by a
greater amount then than during the middle of the day (assuming that
the body temperature at which heat stress causes fatigue is constant).
Indeed, the fact that core temperature starts lower in the earlier part of
the daytime accounts for the observation that self-chosen work rate can
be better sustained at this time in exercise exceeding 60 min in duration,
when core temperature is rising from a lower starting point (Atkinson
and Reilly, 1996).
Instead of considering that the circadian rhythms of sports perform-
ance are wholly exogenous in origin, an alternative interpretation would
be to consider that the complex changes required for exercise have both
exogenous and endogenous components, and that the endogenous com-
ponent is a reﬂection of the body clock. This implies that the circadian
response to exercise is similar in origin to the rhythm of core temperature,
and this could account for the general parallelism between the two
rhythms. There is a considerable amount of evidence that the circadian
rhythm of core temperature results from changes in the “set-point” of
heat loss mechanisms (summarized in Aldemir et al., 2000; Waterhouse
et al., 2004), and that these can be considered to be reﬂections of the
activity of the body clock. It seems likely that similar differences with
time of day might exist in the central and peripheral components of the
response to exercise. Since the constituents of athletic performance—
metabolism, the cardiovascular and respiratory systems, muscle strength,
Circadian Rhythms in Sports Performance 37
and the control of movement by the central nervous system—have all been
shown to have some endogenous component, even though this might be
rather small on some cases, it is illogical to consider that athletic perfo-
mance might possess no such component.
Jet Lag: Impact and Mechanisms
Whatever the detailed nature of the origin of the circadian rhythmicity
of physical performance, unless it contains no endogenous component
whatsoever, problems in sports performance would be predicted to arise
in the days immediately after a time-zone transition due to the slow adjust-
ment of the body clock to a change in local time (Waterhouse et al., 1997).
As a result, the normal synchrony between the endogenous and exogenous
components of circadian rhythms (see Figure 4) will be lost until adjust-
ment of the body clock to the new time zone has taken place. The lack of
adjustment of the body clock will mean that activity will be performed at
times that are no longer near to the temperature maximum; for
example, after a ﬂight to Eastern Australia (10 time zones to the east of
the UK), activity at 15:00 h by local time will initially coincide with the
temperature minimum. Also, the inappropriately timed body clock will
mean that sleep will be more difﬁcult and fractionated than normal; for
example, after the ﬂight to Eastern Australia, sleep will initially be
attempted at about 14:00–22:00 h by “body time.”
One of the most obvious effects of these disruptions is that subjects
suffer from “jet lag”. This is characterized by an assortment of symptoms
including fatigue (and yet inability to sleep at the new night time), head-
ache, irritability, losses of concentration and motivation, and gastro-
intestinal disorders including indigestion, loss of appetite, and bowel
irregularities (Waterhouse et al., 1997). The exact interpretation to be
placed on “jet lag” depends on the individuals and details of their
journey (Waterhouse et al., 2002) and also on the time of day when it is
measured (Waterhouse et al., 2003). Thus, in the early morning and late
evening, estimates of jet lag correlate most with aspects of the recent and
forthcoming sleep, respectively; in the middle of the day, by contrast,
the higher correlations are with the perceived falls in motivation and the
ability to concentrate.
Even though many aspects of physical performance appear to be little
affected by sleep loss, for mood, motivation, and mental performance,
sleep loss is more deleterious (Folkard, 1990; Meney et al., 1998; Water-
house et al., 2001). While the excitement of a competition might override
these effects, when the routine of training prior to an event is concerned,
the combination of effects of sleep loss and jet lag is likely to be a negative
inﬂuence. Trainers, coaches, and the individuals themselves must be
B. Drust et al.38
aware of this, and expect training to be more arduous and less rewarding
in the days immediately after a time-zone transition.
Field studies of performance after a time-zone transition have some-
times been unconvincing with regard to showing a deterioration in per-
formance (Youngstedt and O’Connor, 1999). This might reﬂect the fact
that the design of the study and the difﬁculty of making accurate measure-
ments in such circumstances, the exogenous component of the rhythms
(which will have adjusted to the time zone) are very marked, or that any
circadian effects are overridden by the excitement of the moment. The dif-
ﬁculty of introducing a control condition (e.g., a ﬂight of a similar duration
but one which does not cross any time zones) into jet lag studies means that
the design of these studies is at best quasi-experimental (Atkinson and
Nevill, 2001). Single assessments at a particular time of day before and
after the ﬂight (O’Connor et al., 1991) can be misleading due the fact
that the timing of the circadian rhythm will be different at the destination
compared to at home. A test performed at 18:00 h at the new destination
could be at a different time in terms of the body clock. Multiple measure-
ments of performance at the destination (e.g., Reilly et al., 2001; Figure 5)
are therefore essential and would offer greater insight into the relative
endogenous nature of the rhythms.
The fact is that elite athletes do suffer from jet lag (Edwards et al., 2000;
Waterhouse et al., 2000; Lemmer et al., 2002) and there are reports of
FIGURE 5 A quasi-experiment (Reilly et al., 2001) into the effects of transmeridian travel on a
performance variable (leg extension strength). Measurements were obtained four times a day on
four post-ﬂight days. The diurnal proﬁles suggest gradual adjustment of the circadian rhythm to
the new time zone (5 hours difference). Open bars – afternoon work-rate, ﬁlled bars – morning
work-rate, open symbols – afternoon temperature, ﬁlled symbols – morning temperature.
Circadian Rhythms in Sports Performance 39
how to promote adjustment of athletes to time-zone transitions (Cardinali
et al., 2002; O’Connor et al., 2004). Promoting adjustment hardly seems
appropriate unless there exists at least the possibility that performance
will be impaired! In some cases, there is ﬁeld evidence that performance
is impaired and that the circadian rhythms are phased more appropriately
for the time zone just left (Reilly et al., 2001; Figure 5). In summary, there-
fore, while more and better ﬁeld studies are required, chronobiological
considerations and the limited evidence available indicate that physical
performance is likely to be impaired immediately after a time-zone tran-
sition, particularly if it entails routine training rather than a competitive
We have discussed circadian rhythms in sports performance from both
an applied and basic research perspective. From an applied perspective,
the main considerations that inﬂuence whether time of day does affect
sports performance are external validity of the research sample and how
close the selected performance tests mimic real sports competitions.
From a basic perspective, the validity of the study conclusions depends
on how well exogenous factors have been controlled or discounted in
The optimal time for performance will be dependent on the type of
activities required in the sport and their relative importance to the
overall performance (Reilly et al., 2000) as the component circadian
rhythms may peak at different times (Winget et al., 1985). Analysis of
the different rhythms in performance would suggest that the performance
of skill-based sports and those requiring complex competitive strategies,
decisions and the delivery, and recall of coaching instructions is best com-
pleted in the morning. Sports that require substantial physical efforts
should be completed later in the day. The timing of sports that require
both elements is less clear. It should, however, be noted that these sugges-
tions may represent an oversimpliﬁcation of the situation.
There is a wealth of information from both applied and experimental
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