Research indicates that the optimal time to exercise is when your body temperature is at its highest, which, for most people is the late afternoon, between 4 p.m. to 5 p.m. Other studies do confirm that exercisers perform better on physical performance tests, including aerobic capacity, endurance, reaction time, and strength between 4 and 7 p.m.
But before you start changing your running routine to always run in the late afternoon/early evening, keep in mind that there are also benefits to running in the morning. Running in the morning may help keep you motivated to run. Research shows that morning exercisers are more consistent with the workout regimens than those who work out in the afternoon or evening.
And, although you may have a natural tendency to run better in the late afternoon or evening, some research indicates that your body can adapt to the time of day that you normally train. This is another plus for those who prefer to run in the morning, since most races are run in the morning. So, if you're training for a race, do some of your runs, especially long runs, in the a.m. so your body gets used to running at that time. You'll also get accustomed to the routine of wake up, drink, eat, use the bathroom, run -- so it will feel like second nature on race day. Think of your runs as dress rehearsals for the real race.
If you usually run in the morning, you'll also sleep better -- a study found that morning exercise leads to better sleep quality than afternoon or evening exercise.
So, while there seems to be more evidence indicating that morning running is better than running in the afternoon or evening, the bottom line is that some running is better than no running at all. Run when it works best for your schedule because no matter what time of day you run, you'll still get all the great benefits of running.
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"For Best Sleep, Work Up a Sweat in the Morning" American College of Sports Medicine press release, 6/5/11
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Hill D.W., et al. Temporal specificity in adaptations to high-intensity exercise training. Med Sci Sports Exerc. 1998 Mar;30(3):450-5
Reilly T, et al. Diurnal variation in temperature, mental and physical performance, and tasks specifically related to football (soccer). Chronobiol Int. 2007;24(3):507-19.