by Steve Cline

The answer to one question is vital in choosing a time to exercise: When will you be able to do it consistently?

As you try to find a few minutes each day to work out, you may wonder: What’s the best time to exercise? There are some surprising pros and cons to exercising at different times.

Trainers often tout exercising first thing in the morning as ideal. While early birds gain some advantages, research shows you’re not at your physical peak until later in the day.

In each of us, an internal rhythm governs a host of physical systems. We call this roughly 24-hour cycle the circadian rhythm. For the most part, we’re not conscious of this cycle. But you may have run afoul of your circadian rhythm when you feel jet lag. In that case, your internal clock isn’t in sync with the world around you.

Less obvious is the fact that your body temperature changes by as much as a degree and a half through the day. “Pulmonary function, oxygen exchange, blood flow to the muscles — these are different at different times of the day,” says Phyllis Zee, M.D., a professor of circadian biology at Northwestern University. “That makes your capacity for exercise different, too.”

According to Dr. Zee, the body functions best in the late afternoon or early evening — roughly from 2 to 6 p.m. You reap some benefits by working out at this time. Among them: warmer muscles due to a higher body temperature, improved lung function and even a greater tolerance for discomfort.

Dr. Zee helped write a study that found people who exercised slept better than those who did not, and people who exercised later in the day had even more restful sleep. Still, she suggests you exercise at least two or three hours before you plan to go to sleep. Working out too close to bedtime can cause insomnia.

While working out later may help you sleep and perform better, that doesn’t mean it’s the best time for everyone. Dr. Zee suggests you let your body, your level of alertness and your mood be your guide.

“People know whether they are a morning person or an evening person, whether they are a lark or an owl,” she says. “If you are a lark, go ahead and exercise in the morning. If you are an owl, the better time for you to exercise is the late afternoon or early evening.”

Researchers at the University of Surrey in England have found a genetic link between peoples’ fondness for early morning or late evening. “If you have a night owl and you tell that person to work out in the morning, that’s just not going to happen,” says Carl Foster, Ph.D., a professor in the department of exercise and sports sciences at the University of Wisconsin–La Crosse.

Early morning exercisers do have one decided advantage, says Carol Ann Kennedy, professor of kinesiology at Indiana University. “Exercise in the early morning tends to have higher adherence,” she says. “That’s the one time of the day when other obligations don’t interfere.”

Early morning exercise may also be a bit better for your heart, says Frank Booth, Ph.D., a University of Missouri physiology professor. Working out at least 12 hours before a fat-laden dinner can decrease the amount of lipids (fat) in your blood. Here’s why: Vigorous exercise leads your body to draw on the fat stored in muscles for energy. The body doesn’t pull fat back into the muscles for about 12 hours afterward, Dr. Booth says. So, if you eat a high-fat meal 12 hours after exercise, that fat is drawn into the muscles rather than coursing through your bloodstream. Less fat in the bloodstream means less fat causing plaque buildup.

These experts stress that the answer to one question is vital in choosing a time to exercise: When will you be able to do it consistently?

“If you exercise, you’re better off — whenever you exercise,” Dr. Foster says. “Even though exercising at a certain time may be 1 percent better, you need to find a time that fits for you as an individual and stick to it.”

Warm Up to Your Workout
Most people’s body temperature peaks between 2 and 6 p.m., but you can get a more exact read on your circadian rhythm.

Take your temperature every couple of hours for five to six straight days, the American Council on Exercise suggests. A digital thermometer can give you a pretty good reading. You may find that it’s easier to exercise within a couple of hours of the time your body temperature peaks each day.

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