What’s the Best Time of Day to Exercise? The Answer Is Complicated

7 to 9 a.m.: Pain tolerance is higher.

Some studies have found that people are less sensitive to induced pain in the early morning than later in the day. No one knows exactly what's behind this effect, but researchers from the University of Haifa in Israel speculate that it may have to do with the lingering analgesic (a.k.a., pain-killing) effects of hormones (such as melatonin and cortisol) that peak during the night. This means "your ability to tolerate discomfort [related to exercise] may be higher in the early morning," notes Dr. Thomas Rowland, a pediatric cardiologist at the Baystate Medical Center in Springfield, Massachusetts, and author of "The Athlete's Clock."

The downside of exercising early in the morning: Your flexibility is poor, partly because your body temperature is low within 90 minutes of awakening, Breus says. "When your body temperature is low, you'll be stiff, and this is the time you're most likely to sustain an injury doing any type of physical activity."

10 a.m. to 12 p.m.: Alertness, reasoning skills and short-term memory peak.

This may be related to enhanced glucose metabolism, Breus says, "which is what mental strength – attention, focus and the like – is all about."

In a 2005 review of the medical literature, researchers from the Research Institute for Sport and Exercise Sciences at Liverpool John Moores University in the U.K. concluded that "the performance of skill-based sports and those requiring complex competitive strategies, decisions and the delivery and recall of coaching instructions is best completed in the morning."

So this may be the optimal time to learn and practice complex dance moves, football or soccer plays, or other sports-related skills that involve strategy and decision-making.

4 to 8 p.m.: Overall exercise performance peaks.

In physical activities involving strength, power, speed and stamina, this is the time zone when you're likely to do your best, possibly because it coincides with your peak body temperature. "When your body temperature is higher, you'll have greater lung capacity, blood flow to muscles and flexibility," Breus says. Your hand-eye coordination and reflex reaction time are better now, too. So this is a great time for running, cycling, swimming or playing racket sports (tennis, anyone?), as well as team sports. A 2007 study from the U.K. found that soccer-specific skills, like dribbling speed, also peak in the early evening. So does performance during a cycling time trial, according to a 2005 study by the same researchers.

In addition, ratings of perceived exertion, or RPE, are at their lowest in the late afternoon and early evening hours, Rowland notes, which means you may be able to exercise harder for longer.A 2014 study from the University of North Texas in Denton found that healthy young men were able to perform exhaustive severe-intensity sessions on a stationary bicycle for 20 percent longer (until they reached the point of exhaustion) between 5 and 8 p.m. than between 6:30 and 9:30 a.m.

Keep in mind, however, that exercising too close to bedtime can compromise the quality of your sleep. It's best to complete physical activity within three hours of turning in, Breus says, to avoid having trouble falling asleep. So if you typically turn out your lights at 10 p.m., quit exercising by 7 p.m. so that your body temperature has plenty of time to drop, setting the stage for good-quality shut-eye by bedtime.

Remember, too, that while these optimal time zones apply to many people, those who are early-morning types or late-night types may find considerable variation – as much as three to four hours – in either direction. (Also, traveling across time zones and doing shift workcan throw your body's circadian rhythms off track.) "The only way to know how these effects influence you is to self-experiment," Rowland says. "Track how your performance varies throughout the day" and then try to schedule your exercise training sessions for when you're likely to be at your personal best.

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