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The question

When is the best time of day to work out?

The answer

In addition to facing the unbeatable Michael Phelps, swimmers at the 2008 Olympic Games faced an unusual challenge. To accommodate NBC's scheduling demands, medal finals were held starting at 10 a.m. Beijing time instead of the usual evening timeslot - a change that required extensive preparation.

"Swimming Canada changed the entire season's schedule to emphasize swimming fast in the morning," recalls Gordon Sleivert, head of the national squad's sports science team.

Most people are significantly faster and stronger in the late afternoon and early evening than they are first thing in the morning. You can take advantages of these daily fluctuations by planning workouts for when you're strongest - or, for those who exercise early in the day, using some of the techniques adopted by Canadian swimmers in 2008 can minimize the effects of working out at the "wrong" times.

Circadian rhythms are governed by an internal clock controlled by the "suprachiasmatic nucleus" in the hypothalamus, which receives light signals directly from the retina. This clock governs cyclical rhythms throughout the body, influencing 24-hour patterns of temperature and hormone secretion as well as sleep and feeding cycles.

A 2007 study by Tunisian and French researchers found that power in an all-out 30-second cycling test was lowest at 6 a.m., then increased steadily through the day until it was about 10 per cent higher at 6 p.m., then fell steadily. A long list of earlier studies had found similar effects in back and arm strength, vertical and broad jump, and also in sports ranging from swimming to badminton, with the peak time always within a few hours of 6 p.m.

This effect may be partly a function of time awake (you're groggy early in the day and tired later in the evening) and eating patterns (you won't be at your best before breakfast or immediately after lunch).

But more subtle circadian rhythms, such as the daily change in core body temperature, also play a role, Dr. Sleivert says. Studies have found that body temperature rises by about 1 C between early morning and late afternoon, which may help loosen muscles and swell blood vessels in the same way a pre-exercise warm-up does.

In support of this theory, researchers from Guadeloupe published a study earlier this year in which they tested maximal muscle power in two groups of athletes training either in the morning or the evening. Unlike several earlier studies, they found no difference between the groups after five weeks. The researchers attributed this to the "passive warm-up" provided by Guadeloupe's hot, humid climate, which made the body's internal temperature fluctuations irrelevant.

This trick won't help in Canada's climate. But Canadian swimmers did adjust their warm-up routine to prepare for morning finals in 2008, rising before breakfast to swim a short warm-up and then doing a second warm-up before their races to compensate for lower core temperatures.

The same approach could help anyone working out in the morning. If you usually jog or cycle for five to 10 minutes to warm-up, for example, try adding an extra five minutes and accelerating for the last few minutes to make sure you break a sweat and get your heart rate up before starting your workout.

It's also possible to subtly change your circadian rhythms, Dr. Sleivert says, "and the biggest thing that affects them is intense exercise."

Studies since the 1980s have found that if you usually train in the morning, you'll see greater improvements when you're tested in the morning rather than the evening; the reverse is true if you usually train in the evening.

In 2007, Finnish researchers found that the daily fluctuations of the stress hormone cortisol (which inhibits the muscle-building effects of testosterone) were different in volunteers who lifted weights in the morning compared with those lifting weights in the evening, showing that the choice of training time does indeed alter circadian rhythm.

Canadian swimmers preparing for Beijing timed their hard workouts to coincide with expected race times, and major pre-Olympic meets switched to morning finals. You may do the same thing if you're preparing for a 10K race that starts at 7 a.m. by making sure to schedule key training sessions for that time.

For most people, the best time to work out will still simply be whenever they can find the time and energy. Fortunately, these findings suggest that, if you stick to a consistent routine, your body will eventually adapt to whatever workout time you choose.

Alex Hutchinson blogs about research on exercise at