Here's news for that perniciously perky aerobics instructor: your body's internal clock may influence how well you work out at different times of the day. Although anecdotal observations have long suggested such a link, an international team of researchers quantified the connection in a new study, which they will present this Friday at the Endocrine Society's annual meeting.

The researchers studied 40 healthy men between 20 and 30 years old, timing their melatonin secretions to determine the settings of their internal clocks. (Hormones such as melatonin, cortisol and thyrotropin help set the body's daily clock, or circadian rhythm.) They then divided the men into five groups and asked them to exercise in the morning, afternoon, evening, night or not at all. While the test subjects worked out on a stair-stepper for an hour, the researchers measured their blood levels of melatonin, cortisol and thyrotropin, as well as growth hormone and glucose. Next they compared the blood levels with those measured in the control group that rested.

"We found strong evidence for substantial changes in glucose metabolism and an array of hormonal responses to one-hour, high-intensity exercise, dependent on the timing of the exercise," lead author Orfeu Buxton of the University of Chicago says. In particular, they discovered that levels of cortisol and thyrotropinhormones important for energy metabolismrose the most during evening and night exercise sessions. At night, though, glucose levels also dropped steeply.

"The effects of exercise we observed may explain how some times of day could be better than others for regular exercise or athletic performance," Buxton says. "There is the possibility that evening exercise could be a better time because we see greater effects at that time of day than others, but that remains to be determined by future studies."