The Best Time of Day to Exercise

Q: When is the best time of day to exercise? I am planning to start and can only fit in a walk on the treadmill in the evening. Will this make it hard for me to get to sleep?

A: It seems logical to assume that if you rev your body up at a time when it's naturally starting to wind down, you may find it hard to get to sleep. But there are many factors that can affect how your body reacts:

• How much sleep you've been getting over the past days and weeks (are you sleep deprived or well rested?)
• How hard and how long your exercise session is
• How late in the evening your workout is

Your personal response may vary. People who consume caffeine report different individual responses, too. Some say that having a soda or coffee in the afternoon or evening will keep them up all night, while others down shots of espresso and claim no disrupted sleep whatsoever.

Sleep research is a relatively new science and there's still much to learn. The body operates on a 24-hour clock, and every cell in every physiological system is guided by what are known as circadian rhythms. As the sun goes down, certain systems in the body (such as digestion) tend to slow down, while those processes that occur during the resting hours (such as cell repair) tend to ramp up. Certain triggers can throw off the body's clock. Stimuli such as electric lights and TV can keep the body aroused and cause changes in circadian rhythms that may make it harder to get to sleep.

It appears that exercise can affect circadian rhythms, too. One study tracked changes in the levels of melatonin, a hormone that helps regulate the body's sleeping and waking cycles, which normally peak at night. Published in the American Journal of Physiology–Regulatory, Integrative and Comparative Physiology, the 2003 study found that young adults (ages 20-32) and older adults (ages 55-73) both experienced delays in rising melatonin levels following late-night exercise. Researchers measured what's known as the "dim-light melatonin onset," a marker used to determine the cycles within a person's 24-hour body clock.

In this study, subjects performed three hours of light-to-moderate intensity cycling, and they didn't begin the exercise until quite late at night—around 10:30 p.m. Subjects were then allowed to sleep for only six hours the next day. Subjects were monitored for only a three-day period, and the researchers did not track their ability to go to sleep the night after the study ended (when they were feeling the effects from having had only six hours of rest).

This raises a question: If they exercised again the following evening, would they be so tired that the subsequent exercise wouldn't have much of an effect on their ability to fall asleep? As with many studies, there are still many unanswered questions, and it's unclear what the longer-term effects might be from following a late-night exercise schedule. Of course, if you're doing a 45-minute walk at 8 p.m., the effects may be much less noticeable (though everyone is different).

A considerable amount of research on jet lag and people who work night shifts has looked at the different triggers for disrupting and resetting a person's body clock. A review of the research in a 2007 issue of the European Journal of Applied Physiology noted that exercise is associated with small phase delays in body rhythms. But the researchers noted that there was still much to learn about exactly how much exercise or what intensity of exercise might affect different people. There is some evidence that regular, moderate exercise may even help synchronize people, including those who travel between time zones or work night shifts. But this may also depend upon the time of day that the exercise is performed, and other variables may play a role, too. For example, if a person exercises outside in the morning or evening light, or indoors in a lab where the light is either dimmed or bright and fluorescent, is it the exercise or the light stimulus that affects circadian rhythms?

From a practical perspective, getting adequate exercise and sufficient sleep are both vitally important to your health. If the only time you have to exercise is in the evening, and if you also find that you toss and turn in bed after a workout, then experiment with different times, durations and intensities of your workouts to see if you can find a regimen that doesn't keep you up at night.

Find all articles by Martica.

More From MSN Health & Fitness:

• Wake Up, Work Out
• What Your Sleep Personality Says About Your Waking Life
• 12 Surprising Things That Are Making You Tired
• Bing: Exercise and Health

Do you have a fitness or weight-loss question for Martica? Send e-mail to experts@microsoft.com. Please include Ask Martica in the subject line. Each of our experts responds to one question each week and the responses are posted on Mondays on MSN Health. We regret that we cannot provide a personalized response to every submission.

Martica Heaner, Ph.D., M.A., M.Ed., is a Manhattan-based exercise physiologist and nutritionist, and an award-winning fitness instructor and health writer. She has a Ph.D. in behavioral nutrition and physical activity from Columbia University, and is also a NASM-certified personal trainer. She has written hundreds of articles for publications such as Self , Health , Prevention , The New York Times and others. Martica is the author of eight books, including her latest, Cross-Training for Dummies. (Read her full bio.)