Put off that workout

GET up at 5am to exercise like crazy. Sure, a pre-dawn workout comes with some bragging rights - just don't expect your best performance. A new study suggests that late night, not early morning, is the best time to exercise, as dictated by circadian rhythms. These rhythms affect the daily production of hormones, brain activity, body temperature and workouts. Previous studies found that exercise performance generally peaks later in the day and is worse in the morning.

In case other factors such as stiff muscles, hunger and morning grogginess - instead of circadian rhythms - could have skewed the results, Christopher Kline, a Ph.D. student in exercise science at the University of South Carolina, and co-workers created a more level playing field in their study, which appears online on the website of the Journal of Applied Physiology.

The 25 subjects - 13 women and 12 men between the ages of 16 and 35, all highly trained swimmers - spent four weekends sequestered in the lab, with schedules regulated to two hours of wakefulness, followed by one hour of sleep, to distribute the amount of sleep evenly over a day. They had similar meals, and their exposure to light (which affects circadian rhythms) was controlled.

The swimmers hit the pool every nine hours for a 200m freestyle swim. Their times were generally faster later in the day - fastest at 11pm and slowest at 5am, with a time difference of 5.84 seconds. That's significant in competitions such as the Olympics: 'Even a hundredth of a second is of paramount importance,' Kline says.

The participants' sleep schedule was unusual, but he doubts it radically skewed results. Short amounts of sleep, he says, have been shown in other studies not to affect anaerobic performance. The swimmers' varied schedules allowed them to have their first trial at various points in the day.

The physiology behind the performance variation is unclear. But he notes that circadian rhythms cause body temperature to be lower in the morning and higher later in the day. A higher temperature, he says, prepares the body better for anaerobic and strength training. The rhythms can also affect mood, which is tied to athletic performance.

These results could help professional athletes know when their performance would peak and dip.

'If someone goes to a (foreign country) two weeks before a competition to get acclimatised - which is a standard recommendation to avoid jet lag,' Kline says, 'they may actually be setting themselves up to do their worst performance.'

An athlete might do better at a morning competition, for example, if the body hasn't acclimatised and still thinks it's the afternoon.

LAT-WP