image for exercise article Exercise physiologists have found some differences between athletic performance in the morning and later in the day. Many of these differences are attributed to the body’s circadian rhythms—24-hour cycles that control physical and behavioral factors such as sleep, mood, metabolism, and body temperature. These cycles are driven by signals from the brain and other organs in the body.

Improving Fitness and Mood

If you are looking to improve your fitness, lose weight, enhance your mood, and/or just have fun, the best time to exercise is the time that you will consistently do it. The differences measured in research labs will likely have little bearing on your enjoyment and fitness level. In fact, researchers who looked at decreased anxiety and improved mood found that exercise at any time of day was equally as effective.

In addition, researchers from England observed seven men riding a stationary bicycle at a “submaximal” exertion level. The men's performance was not affected by the time of day. Findings were reported in the Ergonomics journal.

Shaving Seconds

For more competitive athletes, the time of day you do your most vigorous workouts does appear to have an effect. In a study published in the Journal of Sports Science, researchers studied nine male runners who ran at lactate threshold (the maximum intensity at which steady-state exercise can be maintained). The run lasted for 30 minutes at two different times—between 7:00 am and 9:00 am, and between 6:00 pm and 9:00 pm. Body temperature was lower in the morning. In addition, perceived exertion (the runner’s assessment of how hard the exercise felt to them) was higher during the morning run. This study and others have identified body temperature as an influential factor.

Researchers believe that workouts are most productive when body temperature is at its highest, which is between 2:00 pm and 6:00 pm. Body temperature is at its lowest 1-3 hours before waking up in the morning, and gradually increases throughout the day. This increase is small—only about 1-2 degrees—but appears to be enough to boost muscle flexibility and strength. Researchers have also associated the higher temperature with an easier feeling workout (a lower perceived exertion score).

The difference between morning and afternoon exercise bouts are not all due to body temperature, according to a study in the Journal of Strength and Conditioning Research . In this study, researchers from the University of Arizona at Tucson engaged 10 competitive swimmers in four different swim tests, in the morning and in the afternoon, and with a short or long warm-up. The longer morning warm-up was successful in raising body temperatures to match those measured in the afternoon. But even with equal body temperatures, performances were still better in the afternoon.

The Best Time for You

How does all this research relate to the average athletic person? If you do anaerobic or speed workouts (on the track, in the pool, or in the gym; and usually do so just once per week), you will likely get more out of them if you do them in the afternoon. But for less strenuous workouts, the time of day has no effect on performance.

Therefore, the best time is:

  • When you can fit it in—Strive for at least 30 minutes of exercise on most days of the week. People who exercise in the morning are typically more faithful to their exercise routine. Often as the day goes on, responsibilities mount up, time gets short, and exercise drops off the list.
  • When your partner can do it—People who workout with a partner are more likely to stick with their routines. Knowing that someone is depending on you makes you more accountable. And having someone to talk to makes it more enjoyable.
  • When you will be racing—If you are training for a marathon that starts at 7:00 am, begin all your long training runs at 7:00 to help your body become accustomed to exertion at that time of day. A study in the International Journal of Sports Medicine suggested that consistent training at a particular time of day can lead to adaptations in circadian rhythms.
  • When you need an energy boost—Morning exercisers enjoy a jump start to their morning, while those who work up a sweat in the afternoon can avoid the post-lunch slump. Keep in mind that working out too close to bedtime can make it difficult to fall asleep.
  • When weather permits—If you exercise outside, be careful in extreme weather. In the summer, exercise in the morning or evening to avoid the mid-day heat and humidity. In the winter, heed wind chill advisories.

You will reap many physical, emotional, and social benefits no matter what time you exercise. Find a time that works for you and make it a habit. Start every session with several minutes of warm-up—fast walking or jogging and light stretching—regardless of the time of day.