I've read many anecdotes about exercise improving sleep, but research seems to show something else:
In a representative study that he led several years ago, for instance, college students — some athletic, some sedentary — kept detailed sleep and exercise diaries for months. At the end of that time, the researchers cross-referenced the diaries and found no notable correlation between exercising more and sleeping better or vice versa. Meanwhile, in a second part of the same study, a group of adults wore monitors that recorded their movements and sleep patterns. The participants also filled out activity diaries. Using the objective data from the monitors, together with the diary reports, the researchers found only marginal impacts on sleep from exercise. The most active volunteers tended to fall asleep about a minute and a half faster than those who were the least active. Otherwise, their sleep was virtually identical.
In a representative study that he led several years ago, for instance, college students — some athletic, some sedentary — kept detailed sleep and exercise diaries for months. At the end of that time, the researchers cross-referenced the diaries and found no notable correlation between exercising more and sleeping better or vice versa. Meanwhile, in a second part of the same study, a group of adults wore monitors that recorded their movements and sleep patterns. The participants also filled out activity diaries. Using the objective data from the monitors, together with the diary reports, the researchers found only marginal impacts on sleep from exercise. The most active volunteers tended to fall asleep about a minute and a half faster than those who were the least active. Otherwise, their sleep was virtually identical.
From http://well.blogs.nytimes.com/2010/05/26/phys-ed-does-exerci...