Chronobiology and Sports Performance
Thomas Reilly, United Kingdom |
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Abstract
Chronobiology is the scientific study of biological rhythms which embraces
ultradian, circaseptal, circamensal and seasonal rhythms as well as
circadian aspects. Circadian rhythms refer to recurring biological cycles
over each solar day. Body temperature is regarded as a fundamental variable,
having a very strong endogenous component. There is evidence that many
human performance measures vary throughout the day in close phase with
the temperature curve. Exceptions apply to endurance performances and
exercise in high ambient temperatures.
The time of day in which sports events are held therefore
may influence the quality of performance. There are considerations also
for safety and effectiveness of training. Circadian influences are most
obvious when underlying rhythms are disturbed, such as in nocturnal shiftwork,
sleep deprivation or desynchronisation of rhythms following long-haul
flights.
Contemporary elite athletes travel regularly across
multiple time zones, for purposes of training or competition. Such itineraries
induce travel fatigue, including jet-lag when multiple time zones are
crossed. Jet-lag refers to a syndrome or cluster of symptoms that reflect
desynchronisation of the normal circadian rhythm. Until rhythms are re-synchronised
to harmonise with the new local time, performance may be compromised.
Jet-lag is more severe and lasts longer the more
time-zones that are crossed. Circadian resynchronisation is faster after
travelling westwards compared to travelling eastwards. In order to hasten
the adjustment to the new local time, the traveller may choose between
pharmacological and behavioural strategies. The former includes benzodiazepines,
central nervous system stimulants, melatonin, caffeine and other putative
chronobiotics. The latter includes alterations of the sleep-wake cycle
prior to travelling, but a behavioural strategy must be geared towards
the direction of flight, the number of time zones crossed and the time
of arrival in the new time zone. Experience of previous long-haul flights
seems to provide opportunity for developing appropriate means of coping
with jet-lag.
1. Introduction
This overview is about chronobiology and sports
performance. I hope to explain what the science of chronobiology entails,
consider the evidence for rhythms in sport performance and give some examples
of how these are manifest. The review must start with some definitions:
Chronobiology represents the study of biological
rhythms. These may be of different lengths – circadian, circamensal,
circannual and so on. Circadian rhythms refer to recurring cyclical changes
with the solar day. Biorhythms refer to three periods of 22, 28 and 33
days length respectively but this theory has no scientific basis.
A cycle is styled by cosinor analysis. A circadian
cycle has a period 24 h in length, an acrophase at time of peak, and a
trough 12 hours later (see Figure 1, top). A biological marker is the
rhythm in rectal temperature which shows a peak close to 18:00 hours before
it starts to decline as night-time approaches (Figure1, bottom).
Figure 1. Nomenclature used in rhythm characteristics
(top) and the circadian rhythm in rectal temperature (bottom).
The main mechanisms responsible for the “body
clock” are the suprachiasmatic cells of the hypothalamus –
this is the cerebral area also responsible for regulation of body temperature
and it has receptors for melatonin. This hormone is secreted by the pineal
gland which is produced in darkness and inhibited by light. The optic
nerve links perception of light to these cerebral structures. The visual
receptors that synchronise the body clock are independent of the classical
cones and rods.
Consequently, the existence of circadian rhythms
is attributed firstly to endogenous rhythms, the so-called body clock.
These are linked to the sleep wake cycle, influenced by the pineal hormone
and its related neurotransmitter serotonin, body temperature and local
timekeepers identified in muscle and other peripheral tissues.
External factors that lock the rhythm into a 24-hour
period are the alternation of sleep and activity, feeding, external light
and darkness, exposure to UV light and exposure to social factors. These
are the so-called zeitgeibers or time-givers (see Table 1).
Table 1. External signals adjusting circadian rhythm.
1.Pattern of sleep and activity
2.Timing and type of meals
3.Alternation of daylight and darkness
4.Exposure to UV light (e.g. direct sunlight)
5.Exposure to social influences
The two sources of observed rhythms can be separated
by living in a constant routine of unchanging temperature and no physical
activity. The result is shown in the changed shape of the curve of rectal
temperature and urine volume (Figure 2). It is clear that there remains
a classical cosine function in both variables, when the effects of activity
are removed. The other approach is to use “forced desynchronisation”
which we can do in an isolation chamber within our premises at Liverpool
John Moores University. When we do so, it is apparent that rising times
and retiring times get progressively later, the body clock regulating
its day around a 26-h period and not an exact 24 hours (Figure 3).
Figure 2. The circadian rhythms in body temperature
(top) and urine flow (bottom). For each variable the difference between
the two curves represents the exogenous component of the rhythm.
Figure 3. A constant routine facilitates an extension
of the circadian rhythms to > 24 h.
2. An endogenous rhythm in performance
Evidence for a circadian rhythm in sport performance
was provided by Atkinson et al. who showed that cycling performance over
a 16.1 km time trial in the morning was improved by light exercise at
that time the morning before compared to change in performance at mid-day.
This design avoided the criticism of looking at personal best performances
owing to a bias in scheduling record attempts.
With the difficulty of manipulating competitive events
for experimental purposes, researchers have resorted to investigating
components of performance. Rhythms are evident over a range of measures
including grip strength, leg strength, flexibility and self-chosen exercise
intensity (Reilly et al., 1997). The latter is relevant to training since
it indicates athletes will naturally pace themselves at a higher exercise
intensity later in the day.
Rhythms in metabolism persist under exercise conditions,
the rhythm in ventilation being stronger than that of O2 .
The rhythm in the ventilation equivalent of oxygen at light and moderate
exercise intensities is shown in Figure 4. The rhythm in O2
does not show up at O2 max , providing that O2 max
is reached. Subjects not displaying evidence of a maximal value were recalled
for testing, which is a good incentive to continue exercise to voluntary
exhaustion. The data show an obvious error when submaximal heart rate
is used to predict O2 max (see Figure 5) or if it is employed
for training prescription at different times of day.
Figure 4. The ventilation equivalent of oxygen shows
a typical circadian rhythm at two submaximal exercise intensities.
Figure 5. Estimating O2 max from submaximal heart rate is prone to
error. The broken line indicates O2 max actually measured.
The strongest evidence for an endogenous rhythm in
sports performance comes from two studies of swimmers. These performers
habitually train in the morning. Yet power output on an isokinetic swim
bench (Reilly and Marshall,1991), and performance in both 100 m and 400
m swims (Baxter and Reilly, 1983) were better in the evening than in the
morning.
Further evidence is that the reduced performance
in the morning is not due to a warm-up effect. Irrespective of whether
cyclists warmed up or not, their times were better in the evening than
in the morning.
The last piece of evidence comes from conditions
that are more or less constant but under sleep deprivation. Soccer players
performed for 92 hours (Figure 6). There were recurring cyclical changes
in activity and in the heart rate responses (Reilly and Walsh, 1981).
Figure 6. Cyclical changes in activity over four days of 5-a-side soccer.
3. Some qualifications
There are a few caveats to guard against a simplification of the relationship
between time of day and performance. First, the menstrual cycle and
time of day may interact. This interaction influences both core temperature
(which is elevated in the luteal phase) and performance which has a
reduced amplitude in the follicular phase (Bambaeichi et al., 2004).
Secondly, the temperature response to exercise may
vary with time of day. Hence there is a greater rise in temperature in
the morning than in the evening. This is manifest in self-chosen exercise
intensity, where the pace is gradually increased with exercise duration
in the morning (Reilly and Garrett, 1998). Therefore, starting marathon
races in the morning may actually be an advantage.
A final association here is the combination of components
in complex skills such as tennis and factors dependent on both the body
temperature curve and on the central nervous system (Figure 7). Whilst
power may be best later in the day, accuracy is best in the morning (Atkinson
and Spiers, 1998).
Figure 7. Serve accuracy and serve velocity in tennis at three different
times of day.
4. Desynchronisation
So what happens when rhythms are desynchronised? This disruption occurs
on travelling across multiple time zones, an opportunity for a natural
experiment. The effect known as jet-lag contrasts with travel fatigue,
the result of travelling north or south directly.
There can be some confusion about the issues of travelling
athletes, even amongst international bodies. In a recent position statement
of FIMS, there is doubt expressed about the existence of circadian rhythms
in exercise performance. Yet recommendations are made on the purported
basis of circadian principles. Similarly, some international athletics
coaches refused to accept the existence of jet lag five years ago before
the Sydney Olympics. This year, to explain the 400 m performance in the
UK Olympic trials, jet-lag was deemed publicly to be the cause of a favoured
athlete finishing 5th only.
So what is the root challenge to the body clock posed
by travelling across time zones? We can compare a trip shown eastward
and westward from London, the base of Greenwich Mean Time. When travelling
east the urge to sleep is displaced to later in the day, so the body clock
needs to advance to match local time (Figure 8). In contrast, flying westwards
requires a phase delay.
Figure 8. The urge to sleep is displaced after
travelling across time-zones.
Prior to the 1996 Olympics, the U.K. used Tallahassee,
Florida as its base. Crossing entailed a 5-hour time-zone transition.
In studies of the gymnasts (Reilly et al., 2001), it was clear that the
performance curve was shifted to the left and returned gradually towards
its reference levels (Figure 9).
Figure 9. Restoration of muscle strength over seven days following
a flight westwards.
This observation was followed up by a study of the
effects of a chronobiotic drug that might act directly on the body clock.
The argument was that as sleep is an important factor in readjustment,
any substance that would help sleep should accelerate re-alignment. This
led to the selection of temazepam for experimentation.
Those subjects on temazepam derived no added benefit
from the drug. At no stage in the re-adjustment was the temazepam response
better than placebo. The conclusions were that jet lag symptoms recovered,
differed with time of day, were unaffected by temazepam and differed between
individuals (Reilly et al., 2001).
The next Olympic Games proved more difficult –
the question was whether to delay the body clock by 14 h or advance it
by 10 h. Either was possible. A stop-over at Singapore would ease jet
lag symptoms and guarantee a phase advance. It was deemed logistically
too difficult and too disruptive for athletes.
A
study of support staff indicated that if they arrived in the morning and
stayed up, they adjusted by a phase delay. This made the mental adjustment
less difficult for them but accentuated their jet-lag.
An alternative was to consider another chronobiotic,
melatonin. It has opposite effects to light (or perhaps exercise), consolidating
the so-called dark pulse.
Administered according
to guidelines, melatonin proved to be ineffective (Edwards et al., 2000).
That led to a resort to behavioural means.
The behavioural alternative is to use natural signals
provided by light and by exercise. Consider how they might work –
light before the trough in body temperature should delay the body clock,
after this trough would advance it. So when using the Olympic preparation
strategy the plan was to get straight from the airport to bed –
sleep in for two mornings. Linked to this approach was the avoidance of
training in the morning and avoid napping in the afternoon. This behaviour
would anchor sleep in the time zone just departed. This strategy of using
behaviour rather than pharmacology seemed to have worked well.
5. Conclusion
In summing up – the main messages are:
- There is indirect evidence of a circadian rhythm in exercise
performance.
- The interactions between time of day and exercise responses
are complex.
- Adverse effects of desynchronisation can be reduced by basing
behaviour on chronobiological principles.
References
Atkinson, G. and Spiers, L. (1998). Diurnal variations in tennis serve.
Percept. Motor Skills, 86, 1335-1338.
Atkinson, G., Todd, C., Reilly, T. and Waterhouse, J. (2004). Diurnal
variation in cycling performance: influence of warm-up. J. Sports Sci.,
22 (in press).
Bambaeichi, E., Reilly, T., Cable, N.T. and Giacomoni, M. (2004). The
isolated and combined effects of menstrual cycle phase and time of day
on muscle strength of eumenhorreic females. Chronobiol. Int., 21, 645-660.
Baxter, C. and Reilly, T. (1983). Influence of time of day on all-out
swimming. Brit. J. Sports Med., 17, 122-127.
Edwards, B.J., Atkinson, G., Waterhouse, J., Reilly, T., Godfrey, R.
and Budgett, R. (2000). Use of melatonin in recovery from jet-lag following
an eastward flight across 10 time-zones. Ergonomics, 43, 1501-1513.
Reilly, T. and Garrett, R. (1998). Investigation of diurnal variation
in sustained exercise performance. Ergonomics, 41, 1085-1094.
Reilly, T. and Marshall, S. (1991). Circadian rhythms in power output
on a swim bench. J. Swim. Res., 7(2), 11-13.
Reilly, T. and Walsh, T.J. (1981). Physiological, psychological and
performance measures during an endurance record for 5-a-side soccer
play. Brit. J. Sports Med., 5, 122-128.
Reilly, T., Atkinson, G. and Budgett, R. (2001). Effect of low dose
temazepam on physiological variables and performance tests following
a westerly flight across four time zones. Int. J. Sports Med., 2, 166-174.
Reilly, T., Atkinson, G. and Waterhouse, J. (1997). Biological Rhythms
and Exercise. Oxford: Oxford Univ. Press.
Thomas Reilly
Research Institute for Sport and Exercise Sciences
Liverpool John Moores University
Henry Cotton Campus
15-21 Webster Street
Liverpool, L3 2ET
United Kingdom

http://www.icsspe.org/portal/bulletin-january2005.htm
Chronobiology and Sports Performance
Thomas Reilly, United Kingdom
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