Swimmers affected by body clock

The daily (circadian) rhythm exerts a powerful influence over a number of biological functions in the human body, including core temperature, sleep/wake cycles and levels of secreted hormones such as melatonin. However, measuring the impact of this rhythm on athletic performance can be very tricky, because it can be easily masked by other environmental variables, such as previous sleep patterns and training, nutritional and emotional status, and motivation. The laboratory is an ideal environment in which to study circadian rhythm variations, because many of the confounding environmental variables can be removed, and this is just what US researchers have just done with a group of 25 experienced swimmers.The swimmers stayed in the laboratory for 50-55 consecutive hours and followed a three-hour ‘ultra-short’ sleep/wake cycle, involving one hour of sleep in darkness and two hours of wakefulness in dim light, which was repeated throughout the test period. The purpose of this protocol was to distribute any behavioural and environmental masking factors equally across the 24-hour period (so that any differences that were observed would be due to circadian rhythm only).

Each swimmer was asked to perform six maximal effort 200m swim trials, which were timed so that they were distributed equally across different times of day, each trial being separated by nine hours. The swimmers’ temperatures were also taken at regular intervals to determine how their core temperatures varied with the time of day, in particular to find out when the minimum temperature (Tmin) occurred; this is when circadian rhythm theory predicts a trough in athletic performance. The swimming performances were then logged and statistical analysis was carried out to discover whether there was any ‘time of day’ (ie circadian) relationship between the best/worst performances.The results revealed a very significant time of day effect on performance; swimming performance peaked from between five to seven hours before Tmin (approx 11pm), and was at its worst in the period from one hour before to one hour after Tmin (around 5am). The average increase in time from peak to worst performance was 5.8 seconds, or 3.5% difference (significant). This study provides yet more evidence that circadian rhythm can and does exert a significant impact on athletic performance, something that athletes and coaches need to be aware of when planning demanding training sessions or preparing for competitions.

J Appl Physiol 2007; 102(2):641-9

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