NEW YORK (Reuters Health) - Exposure to bright light at night reduces morning body temperature, which could have implications for athletic performance, a UK research team report.

Overheating leads to fatigue, which impairs performance, especially in events lasting more than 20 minutes, Dr. Greg Atkinson of Liverpool John Moores University and colleagues note in the International Journal of Sports Medicine.

"The human body clock could help mediate a 'natural' state of pre-cooling prior to exercise in the morning," they add, given that body temperature is lowest between 4 and 6 a.m. Atkinson and his team theorized that exposing people to bright light at night might push forward the early-morning dip in body temperature, and help keep body temperature low during morning exertion.

To investigate, they had six healthy men run on a treadmill for 40 minutes at 7 a.m. under two different conditions: in one, they had been exposed to bright light on the previous evening between 11 p.m. and midnight, but stayed in dim light between 6 and 7 a.m.; in the other condition, lights were kept dim during the night hour but the men were exposed to bright light in the morning.

The temperature in the room where the men exercised was about 31 degrees Centigrade (88 degrees Fahrenheit).

Nighttime bright light exposure delayed the core temperature trough by about an hour and a half, on average, the researchers found. In the hour before exercise, body temperature was roughly 0.2 degrees Centigrade lower with nighttime rather than morning light exposure, "with evidence that this difference was maintained during exercise."

It also seemed that perceived exertion during exercise was lower with nighttime bright light exposure.

The researchers conclude: "A chronobiologically based light schedule can lower core temperature before and during morning exercise in hot conditions."

This could be particularly important for athletes in this summer's Beijing Olympic Games, they point out, as some important events are scheduled for early morning.

SOURCE: International Journal of Sports Medicine, March 2008.