Body clock affects racing prowess

When it comes to athletic performance, everyone’s a night owl, a new study suggests.

Shawn D. Youngstedt and his colleagues at the University of South Carolina in Columbia erased time-of-day cues in 25 trained collegiate swimmers by keeping them perpetually in low lighting for 2 days at the school’s fitness center. Throughout the study, each athlete adhered to a short sleep-wake cycle: 1 hour of rest followed by 2 hours of sedentary activity. The starting time of the cycle differed among the participants.

Core body temperature, on average, ebbs near 5 a.m. The lowest daily temperature varied slightly throughout the test.

Once every 9 hours, each volunteer was instructed to swim a 200-meter freestyle race at peak speed. Start times varied between 2 a.m. and 11 p.m.

A swimmer typically racked up his or her worst time about 5 a.m., which was within a few hours of his or her daily body-temperature minimum, the researchers report in the February Journal of Applied Physiology. From then on, times improved—peaking at 11 p.m. Differences between an individual’s performance extremes varied by almost 6 seconds and proved independent of whether the swimmer initially had been an early bird or a night owl, according to his or her previous sleep schedule, Youngstedt notes.

Although time-of-day effects might make little difference for well-rested athletes competing close to home, Youngstedt says that they could significantly handicap jet-lagged athletes performing in global competitions. In those instances, the Carolina team concludes, athletes might consider using tricks—such as exposure to bright light (SN: 5/27/06, p. 330: Light Impacts)—to reset their body clocks to coordinate with competition times.