The Claim: Morning Is the Best Time to Exercise

“Without a doubt, exercise at any time of the day beats no exercise at all. But are there physiological advantages to working out in the morning versus evening, or vice versa? In various studies, scientists have found that subjects tend to do slightly better on measures of physical performance — including endurance, strength output, reaction time and aerobic capacity — between 4 and 7 p.m. The explanations are numerous: the body’s temperature and hormone levels peak in late afternoon, making muscles more flexible and producing the best ratio of testosterone (the muscle-building hormone) to cortisol (the hormone that does the reverse). But these variations have only small effects. And much as one can adjust to waking up at the same time daily, studies have shown that the body can adapt to the time of day that you train. In several long-term studies, for example, scientists randomly split people into groups and instructed them to train only in the morning or only in the early evening. In the end, the morning exercisers generally did better on tests of physical performance early in the day, while the evening exercisers did better when tested later. On a practical level, that means that if you plan to run a marathon that starts in the morning, it may be best to schedule your training runs early in the day. In general, research suggests that the ideal time to exercise is late afternoon, though the advantages are slight.”

http://www.nytimes.com/2009/03/03/health/03real.html?_r=1&ref=nutrition