The rhythm of life: why timing matters to athletes

The daily rhythm is deeply ingrained in human physiology and research suggests that athletes ignore this rhythm at their peril

The human race has evolved in and is surrounded by a natural world full of rhythms, and it would be incredible if these rhythms didn’t exert a significant effect on our physiological function and performance potential. In recent years, the field of ‘chronobiology’ – how these rhythms affect human biology has confirmed this is indeed the case.

Everybody is aware of the powerful circadian (daily) rhythm; it is after all what regulates sleeping and waking patterns. However, other rhythms can also impact physiological function, although the magnitudes of their effects tend to be somewhat weaker, which can make some of them rather more difficult to detect against the background of environmentally induced physiological variations. Table 1 summarizes some of these rhythms.

---

Table 1: Biological rhythms affecting human biology

---

Circadian rhythm

The circadian rhythm is the most powerful rhythm affecting humans; as well as the sleep/waking cycle, it affects hormone secretions, body temperature, mental alertness and (as we’ll see) physical performance capacity. The graphs in figure 1 below show the typical daily variations of melatonin, secretion, core temperature, triacylglycerol, alertness and reaction time as a result of the circadian rhythm. As a result of these rhythmic fluctuations, many people experience maximum mental alertness, fastest reaction times and highest core temperature in the late afternoon/early evening period, while the peak in melatonin concentrations in the middle of the night period leads to maximum fatigue/sleepiness and lowest alertness.

---

Figure 1: Typical circadian rhythm-induced fluctuations in the body

NB: Melatonin is the ‘sleep hormone’; triacylglycerols are lipids (fats) circulating in the blood.

---

It’s important to understand that while the circadian rhythm is modulated by environmental stimuli, it’s essentially a ‘free-running’ rhythm; put someone in an isolated, darkened room for a week, where they have no idea what time of day it is, and these rhythmic fluctuations will persist. However, in these free-running conditions, the circadian rhythm is not exactly 24 hours, but a little over⁽¹⁾. Thus the need for external stimuli such as daylight to keep the rhythm synchronized with the 24-hour clock. It’s also true that within the basic circadian rhythm, different people may exhibit slightly differing physiological and behavioral responses for example, a ‘lark’ or ‘owl’ tendency (see box 1 and table 2 below).

---

Box 1: Lark or owl?

 The circadian rhythm does not result in identical physiological and behavioral responses in everyone. You probably know of people described as ‘larks’; getting up early and feeling sharp and in good spirits first thing in the morning seems to come naturally to these people, while they tend to wind down in the evening and retire to bed fairly early too. ‘Owls’ on the other hand are night people, who often struggle to awaken, and then feel groggy and perform poorly for hours after waking. However their mood steadily rises through the day and by the evening they’re full of energy.

Physiologists believe that these differing characteristics are primarily related to core temperature patterns; table 2 below shows typical heart rate/core temperature trends for both types. However, pure lark or owl behavior represents the extremes and most of us lie somewhere in between. Moreover, research indicates that pure lark or owl characteristics may not be innate, but are likely to indicate circadian ‘dysrhythmia’ – a disruption to the normal circadian rhythm⁽²⁾. Extreme owl behavior arises from delayed sleep phase syndrome (where the circadian rhythm lags behind the 24-hour day), and extreme lark behavior from advanced sleep phase syndrome (circadian rhythm becomes advanced). The current suggestion is that while we may inherit a tendency to slip into lark or owl behavior⁽³⁾, the correct circadian rhythm can be maintained with environmental clues, such as appropriate bright light exposure (more later)⁽⁴⁾.

Table 2: Lark vs. owl characteristics

---

Circadian rhythm and athletic performance

A growing body of evidence suggests that manipulating the timing of training and/or the circadian rhythm can produce significant benefits for athletes. However, one of the problems that has beset researchers studying the effects of circadian rhythm on physical performance is that the magnitude of these effects tends to be small relative to the continual background ‘noise’ of other factors impacting on performance such as other training, nutrition or psychological factors.

Moreover studies of this type necessarily examine physical performance at different points in the rhythm and on different days; unless the sleep patterns remain constant between tests, significant errors can arise. There’s also evidence that the amplitude of these rhythms may be altered by varying exercise intensity, and that other rhythms can interfere with the circadian rhythm (especially the monthly menstrual rhythm in women⁽⁵⁾).

For example, a UK study looked at blood lactate concentrations (a marker of physiological fatigue during endurance exercise) in 11 trained female endurance athletes at rest and during the final stage of an incremental multi-stage test on a Concept II rowing ergometer⁽⁶⁾. Researchers were keen to discover what exercise intensity was required to produce a blood lactate concentration of 4mmol/l at different times of day (06:00h and 18:00 h), at two different phases of the menstrual cycle: the midfollicular phase and the midluteal phase, which typically occur around 10 and 22 days respectively after the onset of bleeding. The results showed that at the midluteal phase of the menstrual cycle, the 4mmol/l lactate threshold occurred at a significantly higher exercise intensity, heart rate, and oxygen consumption than it did in the midfollicular phase. Unusually however, when researchers looked for a time of day interaction effect, none was evident.

Benefits of afternoon/evening training

The results above contrast sharply with a study by the same group of researchers on circadian rhythms and lactate threshold in male rowers⁽⁷⁾. Eleven male athletes followed the same incremental protocol described above, but this time rowing at 0200, 0600, 1000, 1400, 1800 and 2200 hours (on separate days with full recovery in between tests). The researchers found that the oxygen consumption needed to produce a blood lactate concentration of 4mmol/l peaked at 21.39h while the highest heart rate needed to reach this threshold occurred at 20.32h. In plain English, the rowers were able to work most intensely for a given build up of lactate around about 9pm in the evening, which also coincided with their peak core temperature. It also suggested that there was an interaction between menstrual and circadian rhythms in the female rowers above.

More evidence for the link between circadian rhythm and aerobic/anaerobic performance came from a US study that evaluated the effect of the time of day on high-intensity, constant-power cycling performance by both men and women⁽⁸⁾. Fourteen subjects performed the tests both in the morning and the afternoon in randomised order with the load set at 5 watts per kilo of body for the women and 6 watts per kilo for men (intense – a 75kg male would be working at 450 watts!). Compared to the morning, the total work performed was 9.6% greater in the afternoon and this afternoon work was associated with a 5.1% higher aerobic power and a 5.6% larger anaerobic contribution. Moreover, the trend was equally strong in both the men and the women.

Another study looked at anaerobic power developed in 30-second cycling tests carried out at different times of the day⁽⁹⁾. In this study, French researchers looked at the force and velocity of muscular contractions during cycle ergometry of 19 subjects tested at 02.00h, 06.00h, 10.00h, 14.00h, 18.00h and 22.00h on separate days, and how closely correlated to core temperature any performance changes were. The results were as follows:

• Peak core temperatures occurred at just before 18.30h.
• Maximum peak power tended to occur just before 17.30h (7.6% higher than average peak power).
• Maximum mean power occurred at 18.00h (11.3% higher).
• The changes in power output and core temperature were strongly associated (indicating that this was a circadian rhythm effect).

The researchers concluded that athletes could benefit by recording their temperature and timing their bouts of subsequent anaerobic training to coincide with peaks in their circadian rhythm.

Circadian rhythm and sleep loss

Given that circadian rhythms are involved in regulating sleep patterns, do the afternoon/evening performance advantages persist after a disturbance of the sleep pattern? The same French group (above) examined the effect of one night's sleep deprivation on muscular strength using force/velocity analysis in the morning and afternoon of the following day⁽¹⁰⁾. In the sleep deprivation condition, the subjects remained awake overnight, while control condition, the same subjects slept at home, retiring around 23.00h and rising at 0500 hours. Testing was carried out at 06.00h and 18.00h in both conditions. The researchers (as expected) found that the normal circadian rhythm of temperature fluctuation was not affected by the sleep deprivation, and that even after sleep deprivation, maximal and peak power was still significantly higher at 18.00h. However they also found that the expected improvement in power outputs in the evening was not as great after sleep deprivation – ie after 36 hours without sleep, the expected anaerobic performance due to circadian rhythm was diminished.

Strength and circadian rhythm

Research into how circadian rhythms affect performance is not limited to anaerobic power/lactate studies. A UK study on the effects of circadian rhythm on strength found that the time of day affected maximal lifting strength in young female subjects with an 8% increase in maximal strength at 18.00h compared to 06.00h⁽¹¹⁾. However, this effect was only observed in the luteal phase of the cycle; in the follicular phase, there was no discernible effect.

Another study looked at isometric and isokinetic leg strength in eight women during the follicular phase only of the menstrual cycle (to eliminate any menstrual cycle effects), under conditions of both adequate sleep and partial sleep loss⁽¹²⁾. The results showed that the peak torque (force) generated by the leg muscles was between 4.5 – 5.9% higher at 18.00h compared to 06.00h and that the performance rhythms were synchronised with rectal temperature (ie circadian rhythm). Moreover, partial sleep loss did not alter the magnitude or variations in muscle strength with changing time of day.

These results were supported by a French study on circadian variations on strength in men and women⁽¹³⁾. Twelve men and eight women were tested for maximal isometric voluntary contraction force of quadriceps and hamstrings at six different times of the day (02.00h, 06.00h, 10.00h, 14.00h, 18.00h and 22.00h), the order of which was assigned randomly. At each of these times, three trials were performed separated by three minutes recovery, and the highest value recorded. The results were as follows:

• Circadian peaks (highest core temperatures) occurred at 17.29h and 16.40h for males and females respectively.
• Maximum voluntary leg strength occurred at 17.06h in males (increase of just over 2.5%) and 15.35h in females (increase of just under 3%).

Additional evidence that circadian rhythm affects strength also comes from earlier studies. For example, a study by researchers from the University of Dijon on the variation of maximum isometric elbow torque in PE students at different times of day found that peak torque tended to occur at 17.58h, and was nearly 7% higher than the averaged peak torque figure over the whole day. Moreover, when the experiment was repeated and spread out over a period of 6 days, the peak torque figure was calculated to occur at 17.55h – ie just 3 minutes earlier. This led the researchers to conclude that the circadian rhythms affecting muscular activity are remarkably constant⁽¹⁴⁾.

---

Why does circadian rhythm affect performance?

There are a number of possible explanations as to why performance may be enhanced during the hours around the peak of the circadian rhythm, but increased core temperature almost certainly plays a major role. Higher body temperatures result in less viscous blood flow and muscles that are more supple with less energy losses from internal friction, But there’s evidence that increased core temperatures in the afternoon/evening as a result of the circadian rhythm may also help because the body is in more of a ‘heat loss’ mode (something beneficial during vigorous exercise - than compared with early morning ‘heat gain’ mode when core temperatures are low⁽¹⁵⁾.

---

Manipulating your circadian rhythm for performance

Although some early studies reported little effect of circadian rhythm on athletic performance¹⁶, the overall weight of evidence that circadian rhythm significantly affects performance potential in high-intensity aerobic/anaerobic and strength training is fairly robust. The obvious question for athletes and their coaches therefore is how they can they can train in harmony with the circadian rhythm to maximize performance. Here are some suggestions:

• Measuring circadian rhythm – this is best done by the athlete taking their temperature every two hours during a rest day following several days of a normal, regular sleep pattern. The figures are plotted to observe when the peak occurs (normally late afternoon/early evening).
• Scheduling - athletes should try (where possible) to schedule important and/or strenuous workouts within an hour or so of circadian peak; they will almost certainly gain quality over attempting the same workout earlier in the day.
• Early morning workouts - should be performed at a more gentle pace and after a more thorough warm-up to reduce the risk of injury;
• Early workouts = getting up much earlier than usual (ie when the circadian rhythm is in a trough) to ‘squeeze’ in a workout may be counterproductive; the quality of the workout is likely to be poor, the risk of injury is increased and the athlete will of course be losing sleep into the bargain!
• Hot conditions - adaptation to hot conditions during a workout seems to be more efficient near to circadian peak; if hard morning workouts are scheduled in hot conditions, athletes should consume extra fluid during workouts.
• Strength - athletes trying to build strength should time workouts to coincide with their circadian peak; research suggests that late afternoon weight training produces a more favorable post-exercise anabolic hormone profile, with higher levels of testosterone and lower levels of cortisol (a hormone associated with physiological stress and muscle tissue breakdown)⁽¹⁷⁾.
• Unless trying to manipulate circadian rhythm (see below), athletes should try to maintain regular bedtime and waking hours; irregular hours can disrupt circadian rhythm, leading to a generalized drop in performance.

Resetting circadian rhythm

For competition (where the time of the event is usually fixed), athletes may wish to experiment with manipulating their circadian rhythm so that they’re nearer their peak at the time of the event. The same applies when competing abroad in different time zones. Although the circadian rhythm is free-running, it can be ‘reset’ with the help of appropriate environmental stimuli. The most powerful of these stimuli is light, which strikes receptors in the retina of the eyes containing a photo pigment called melanopsin. This causes stimulation of the suprachiasmatic nucleus (SCN), a distinct group of cells located in the hypothalamus region of the brain, and these cells help interpret information on day length and pass it on to the pineal gland (a pea-like structure found near the base of the brain), which then secretes the hormone melatonin in response.

Research indicates that the SCN is particularly responsive to blue light with wavelengths of around 468nm⁽¹⁸⁾. This has important implications for athletes wishing to reset circadian rhythm. During mid-summer months, simply gradually going to bed and getting up earlier or later (see below) will provide a sufficient stimulus because there’s plenty of bright sunlight (containing blue wavelengths) around to stimulate the SCN. However at other times, light therapy may be useful.

Special lightbox devices can provide a powerful and portable source of 468nm blue light, thereby mimicking the effect that exposure to bright midday summer sunshine has. To advance circadian rhythm (ie shift it earlier, needed for example when travelling east), exposure to blue light first thing in the morning is recommended; to delay circadian rhythm (make the peak occur later – useful when travelling west), evening exposure to blue light is recommended. Here are some other tips for manipulating/resetting circadian rhythm:

• Allow approximately one day per time zone (hour) for circadian rhythm adjustment when travelling across zones.
• Adaptation when travelling west (delaying rhythm) tends to be easier (because the free-running circadian rhythm is slightly longer than 24 hours) and may require less adaptation.
• As well as evening light exposure, research suggests that late evening exercise can also help delay circadian rhythm (useful for travelling west)⁽¹⁹⁾.
• Research suggests that when travelling east or preparing yourself for a daytime competition, resetting circadian rhythm is best performed in advance using several smaller steps of 30 minutes over a longer period, rather than fewer but larger steps⁽²⁰⁾. This strategy can be combined with bright light therapy for maximum benefits.

References

1. J Clin Psychiatry. 2005; 66 Suppl 9:3-9; quiz 42-3
2. American Sleep Disorders Association. International Classification of Sleep Disorders, revised: Diagnostic and Coding Manual. Rochester, Minn. 1997
3. EMBO Rep. 2005; October; 6(10): 930–935
4. Sleep1990; 13:354-361.
5. Clin Ter. 2006 May-Jun;157(3):249-64
6. Med Sci Sports Exerc. 2005 Dec;37(12):2046-53
7. Eur J Appl Physiol. 2004 Jun;92(1-2):69-74
8. Can J Sport Sci. 1992 Dec;17(4):316-9
9. Int J Sports Med. 2004 Jan;25(1):14-9
10. Eur J Appl Physiol. 2003 May;89(3-4):359-66
11. Chronobiol Int. 2002 Jul;19(4):731-42
12. 2005 Sep 15-Nov 15;48(11-14):1499-511
13. 2005 Sep 15-Nov 15;48(11-14):1473-87
14. Chronobiol Int. 1997 May;14(3):287-94
15. Chronobiol Int. 2000 Mar;17(2):197-207
16. Int J Sports Med. 1997 Oct;18(7):538-42
17. Chronobiol Int. 2004 Jan;21(1):131-46
18. Biol Psychiatry. 2006 Mar 15;59(6):502-7
19. Am J Physiol Regul Integr Comp Physiol. 2004 Jun;286(6):R1077-84
20. Aviat Space Environ Med. 2006 Jul;77(7):677-86