A question that is often asked by individuals is, "When is the best time of day to exercise?". The answer to this question is dependent upon many factors. Ultimately, the best time to exercise is the best time for you. To make it a lifelong habit, you want to choose a time that you are most likely to exercise. Regardless of the time chosen, you will still reap the health benefits associated with exercise, such as improved fitness and a reduced risk for disease.

When making your decision about the best time to exercise, you need to weigh the pros against the cons. Some points to consider when making your decision include:

1. Exercise done first thing in the morning is subject to fewer schedule conflicts, reduced likelihood of the unexpected, and less feelings of fatigue that may arise as the day goes on.
2. When the weather is hot and humid, morning and evening hours are cooler, making physical activity during these times safer and more comfortable than exercising during the heat of the day. Note that morning time may have a slight advantage over evening time for better exercise performance when exercising in the heat, according to a study published in the January 2009 issue of Medicine & Science in Sports & Exercise.
3. Air pollution is lower before rush hour traffic in the morning than it is later in the day.
4. In the afternoon to early evening hours (3:00-7:00 PM) your body temperature is higher, making it an optimal time to improve muscular strength. In addition, studies have shown that muscle contractility, extensibility, and flexibility are influenced by a higher body temperature, thus reducing your risk for muscular injury at this time. Note, however, that for men, hormonal levels (e.g., testosterone) are greatest in the morning and that these levels increase more with a morning bout of resistance training than from a session performed later in the day. This implies that if a man's goal is to build muscle (e.g., body build), rather than improve strength, he should lift weights before noon.
5. Exercising in the late afternoon may provide you with the best night's sleep according to the National Sleep Foundation. Although body temperature rises with exercise, if physical activity is done in the late afternoon, it will have a chance to drop by bedtime (it can take up to about 6 hours to decrease). A lower body temperature is associated with the state of sleep. Note that exercise should be completed at least 3 hours prior to the onset of sleep to take advantage of the lower body temperature to induce sleep.
6. Exercise after work or in the evening may help to reduce stress built up throughout the day.
7. If you are training for a sports event, it may be best to exercise at the same time of day that the event will take place.
8. Because exercise speeds up your metabolism, energizes you, and makes you more alert, engaging in physical activity in the morning can prepare you for the day ahead.
9. Exercising in the morning on an empty stomach may impair performance. However, exercising with a full stomach can be uncomfortable. Consuming a light snack that is low in fat and fiber prior to exercising will provide energy while decreasing the likelihood of experiencing gastrointestinal distress.
10. It has been shown, from laboratory exercise stress tests to exhaustion, that the time of day of exercise has no effect on time to exhaustion, maximal oxygen uptake (exercise capacity), or heart rate. However, exercise that is of a very high intensity and short duration (when the body relies on the anaerobic system to provide energy) is performed better in the morning than in the afternoon.

Making the decision to exercise is important. Determining when to exercise should rely on what fits into your schedule and works best for your own personal circumstances and goals. As long as the time of day that you choose gets you exercising, it is the right time.

Resources

"Exercise in the Heat is Greatest in the Morning than in the Evening in Man," Hobson, R.M.; Clapp, E.L.; Watson, p.; Maughan, R.J. Medicine & Science in Sports & Exercise. 41(1): 174-180, January 2009.

"Nutrition and Athletic Performance," Joint Position Statement by the American Dietetic Association, Dietitians of Canada, and the American College of Sports Medicine. Medicine & Science in Sports & Exercise. 41(3): 709-731, March 2009.

ACSM Current Comment "Chronobiological Effects on Exercise," Descheres, M.R.

National Sleep Foundation, www.sleepfoundation.org