Research says go ahead and exercise, no matter what the time.
Late-night exercisers can forget the old-school advice about avoiding workouts during the four hours leading up to bedtime. New research suggests that, while exercise does have some stimulant effects similar to caffeine, evening workouts often have no negative effects on quality of sleep. However, if you do have trouble powering down after late-night exercise, try a hot bath, which has been shown to help people fall asleep faster.
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From the publishers of Prevention