Chronic insomnia, defined as difficulty initiating or maintaining sleep, awakening too early in the morning, or nonrestorative sleep, is the most common sleep disorder among adults. Though exercise has long been assumed to improve sleep, surprisingly little research has been conducted on the effect of exercise on chronic insomnia.
Of the handful of studies that have been performed, they suggest that exercise significantly improves the sleep of people with chronic insomnia. The only study that looked at the effects of a single exercise session found that a bout of moderate-intensity aerobic exercise (e.g., walking) reduced the time it took to fall asleep and increased the length of sleep of people with chronic insomnia compared to a night in which they did not exercise (Passos et al., 2010). However, in the same study, vigorous aerobic exercise (e.g., running) or lifting weights did not improve sleep. Similar results have been found for studies that examined the effects of long-term exercise on sleep in adults with insomnia. In these studies, after 4 to 24 weeks of exercise, adults with insomnia fell asleep more quickly, slept slightly longer, and had better sleep quality than before they began exercising (Guilleminault et al., 1995; Passos et al., 2011; Reid et al., 2010).
Although the exact mechanisms are unknown, there are many possibilities for how exercise may reduce insomnia severity. One way may be by the body-heating effects of exercise, especially when performed in the afternoon or later. Exercise triggers an increase in body temperature, and the post-exercise drop in temperature may promote falling asleep (Horne & Staff, 1983). Exercise may also reduce insomnia by decreasing arousal, anxiety and depressive symptoms. Insomnia is commonly linked with elevated arousal, anxiety, and depression, and exercise has strong effects on reducing these symptoms in the general population. Finally, exercise may reduce insomnia by its effects on circadian rhythms (body clock). For people with insomnia due to the timing of their body clock, exercise may shift its timing depending upon the time exercise is performed.