Exercise is one of the most important things you can do to stay healthy. However, there are many critical elements to make it work effectively for you. The single largest mistake I see most people make is that their intensity level is inadequate to obtain sufficient health benefits. Working out harder is something many people avoid. However, there is a simple modification of your exercise program that may provide a major advantage to you if you aren't doing it already, and that is to change your exercise time to the morning.
Morning exercisers had fewer complaints about a bad night's sleep and those who stretched in the morning had somewhat better sleep, a new study found. Women who exercise in the evening, on the other hand, were more likely to be up at night. The researchers suspect that exercise in the morning might set the women's body clocks for a day of activity and a night of sleep, while exercise at night might push back the sleep part of the sleep-wake cycle. They speculate that the stretches might have improved sleep by making the women more flexible and relaxed.
USA Today November 24, 2003