You might hear or read statements like "getting exercise helps you sleep better". Like most simple statements about sleep physiology it is too simplistic to be always true. But we can say it is mostly true. In some situations exercise may disrupt sleep, but in other situations it will make you sleep sounder and reduce sleep latency.
In fact, starting an exercise program is often recommended as an approach to insomnia for people who want to avoid medicines. Getting tired from exercise may somewhat assist people in falling asleep. It is possible to rest the skeletal muscles without sleep, of course, but athletes who train at a high level often report a desire to sleep after a workout. The average person doesn’t exercise that hard.
These are anecdotes. More formal study of the question has failed to find a strong connection between exercise and sleep quality and duration. Researchers at the University of California at San Diego examined sleep and exercise diaries of college students and found "These results fail to support epidemiologic data on the value of exercise for sleep, but are consistent with experimental evidence showing only modest effects of exercise on sleep."
In a journal article called "Epidemiology of exercise and sleep" researchers from the Univerity of South Carolina concluded "experimental evidence has found acute and chronic exercise to exert only modest effects on subsequent sleep."
There it is some evidence that exercise may be a placebo when it comes to sleep latency. By exercising, people think they will sleep better, and by thinking they will sleep better, they actually do sleep better.
Some people seem to sleep better after exercise and some worse. Not only does the effect vary from individual to individual, it varies with the time of day, length of time between exercise and bed, intensity of exercise, food and drink consumed, and fitness level. Attempts to draw generalizations amount the relationship between sleep and exercise have been thwarted by this observation.
A meta-study back in the 90s found that for the population as a
whole, exercise improves the metrics of sleep quality. The amount of
deep sleep, amount of REM sleep, and total sleep time all tend to
increase with exercise, sleep latency tends and fragmentation tend to
Other older studies had suggested no increase in slow-wave sleep (http://www.ncbi.nlm.nih.gov/pubmed/4069348)
A German study of children found that moderate exercise did not substantially change sleep architecture, but intense exercise shifts sleep from light sleep to deep sleep. (http://www.ncbi.nlm.nih.gov/pubmed/17644426) REM sleep time did not appear to be affected by exercise.
There have been some speculation that the increased body heat from exercise may disrupt the sleep cycle. The body’s thermoregulation process and the sleep cycle are connected.
Exercise in the few hours before bed also makes it difficult to get to sleep for many. One of the principles of sleep hygiene is avoiding rigorous exercise during the evening.
See also: Sleep and Human Growth HormoneMore on exercise before bed