On the other hand, if your exercise time varies from day to day, and it's wearing you out instead of pumping you up, you may be interested in the work of scientists who are studying the proverbial internal clock and how to best determine what time of day you should schedule your workouts.

Rhythm: It's not just for dancing
The secret appears to lie in circadian rhythms, the daily cycles that our bodies follow. These rhythms originate in the hypothalamus and regulate everything from body temperature and metabolism to blood pressure.

The rhythms result from the firing rate of neurons. They have conformed to our 24-hour light-to-dark cycle, and may be regulated and re-regulated each day according to the environment.

Warm is better

It is the influence of circadian rhythms on body temperature that seems to yield the most control over the quality of a workout. When body temperature is at its highest, your workouts will likely be more productive; when your temperature is low, chances are your exercise session may be less than optimal.

Body temperature is at its lowest about one to three hours before most of us wake up in the morning, in contrast to late afternoon when body temperature reaches its peak. (To determine your own circadian peak, refer to the box to the left.)

Studies have consistently shown that exercise during these late-in-the-day hours produces better performance and more power. Muscles are warm and more flexible, perceived exertion is low, reaction time is quicker, strength is at its peak, and resting heart rate and blood pressure are low.

Don't fix it if it's not broken

First of all, don't change your schedule if you feel good beginning your day with exercise. Everyone agrees that exercise at any time is better than no exercise at all. In fact, people who exercise in the morning are more successful at making it a habit.

And though it has been suggested that morning exercise may put some people at higher risk for heart attack, further research indicates that there is simply a generalized increased risk of heart attacks in the morning. If your schedule favors an early workout, emphasize stretching and a good warm-up to insure that your body is ready for action.

Other considerations

If stress relief is your goal, exercise always works, all the time. And if you're wondering when it's best to train for an upcoming event, it all depends on what time you'll actually be competing. If an upcoming marathon begins at 7:00 a.m., try training at that time of day.

Though training at any time of day will raise performance levels, research has shown that the ability to maintain sustained exercise is adaptive to circadian rhythms. In other words, consistently training in the morning will allow you to sustain exercise during a morning marathon longer than if you train in the evening.