The 24-hour rhythmic cycle of our bodies, known as circadian rhythm, significantly affects athletic performance, according to just-published research.

Originating from the hypothalamus gland in the back of the brain, circadian rhythm functions as the body's way of regulating energy levels and other bodily processes according to the passage of time.

It has long been speculated that there may be a circadian rhythm involved in athletic performance and previous research had shown that athletic performance varies according to the time of day.

Past studies, however, failed to account for other factors that could cause time-of-day differences. For example, worse performance in the morning could be attributed to hunger, joint stiffness, sleepiness, lower temperatures outside and insufficient warm up time.

In the latest study, however, published in the February 2007 Journal of Applied Physiology, researchers from the University of South Carolina examined 25 highly trained swimmers over 50-55 hour periods, who were following a 180-minute ultra-short sleep/wake schedule, specifically one hour of sleep in darkness and two hours of wakefulness in dim light. In effect, this study design meant that any significant performance differences would have to be caused by circadian rhythm and not other factors.

Each swimmer performed six 200-meter swim trials distributed equally across eight times of day, with nine hours between each trial. Data from the sleep/wake schedules, swim performances, states of sleepiness, physical/mental energy, and physical/mental fatigue and body temperature measurements were collected.

The results?

Swimming performance varied considerably according to time of day. Specifically, swim performance was impaired between 2:00 - 8:00 AM and peak performance was at 11:00 PM. Also, swimmers clearly did better performance in the afternoon and evening, compared to the morning.

These data provide the strongest evidence yet that circadian rhythm does indeed affect athletic performance. The circadian difference between best and worst performance in this study — 5.84 seconds — would have considerable importance in athletic competition. For example, among females competing in the 200-meter freestyle final at the 2004 Olympics, first and third place were separated by only 0.42 seconds, and first and eighth place were separated by only 1.17 seconds. Among the men, 0.61 seconds separated the winner from third place, and 3.69 seconds separated first from eighth place.

The research helps explain why performance declines following time zone travel, which disturbs circadian rhythms. It may also help athletes compensate for this — by knowing the circadian time of peak performance, athletes may be able to manipulate circadian systems so that peak performance rhythms coincide with the time of competition.