In addition to feeling drowsy and irritable with trouble concentrating during the day, chronic insomnia can eventually lead to lowered immunity, dysfunctional carbohydrate metabolism, slowed thyroid hormone production, decreased glucose tolerance, and elevated levels of the stress hormone cortisol. These conditions can accelerate the development of diabetes, high blood pressure, obesity and other problems.
An ideal solution
Regular exercise is always prescribed as one of the three major cornerstones of good sleep hygiene. (The other two are a regular sleep schedule and avoiding stimulants, hard-to-digest foods and alcohol before bedtime.) Exercise is ideal as a preventive measure and a treatment because it's a health-promoting, inexpensive, safe, and simple means of eliminating troubled sleep. So does it work?
In a detailed review of the existing literature on sleep and exercise, the Department of Exercise Science at the University of South Carolina offers a fresh overview of what is currently known about the relationship between exercise and improved sleep patterns.
The literature on the effectiveness of exercise in promoting sleep has limitations. Presently, there is a lack of large-scale studies that rely on objective measures over self-reporting, which is less reliable and may fall prey to widely-held beliefs that exercise promotes sleep.
Second, a causal mechanism can be difficult to pin down; it may be that better sleep leads to a greater willingness to exercise. There are, however, genuine threads of evidence that exercise improves sleep. In most cases, these have been repeated with great consistency across several published studies.
The theory goes
There are several hypothetical mechanisms by which researchers postulate that exercise promotes sleep. Because it is widely believed that sleep serves an energy conservation function, regulates body temperature downward, and allows for body tissue restitution, exercise is the best candidate stimulus to promote it since nothing else elicits greater energy depletion, body temperature elevation or tissue breakdown in humans.
One of the most plausible scenarios in which exercise would promote sleep is through anxiety reduction. It's well documented in the literature that exercise can lower anxiety and also, as the review author writes, that "disturbed sleep is a hallmark of anxiety."
Additionally, chronic insomnia has been associated with increased physiological arousal. As it acts to counterbalance this excess arousal, exercise is a likely candidate on both of these fronts to promote undisturbed sleep. Similarly, the well-established antidepressant effects of chronic exercise may result in better, longer sleep.
What we do know
A new view of a long-held assumption. The literature reflects that the most positive effects occur following exercise four to eight hours before bedtime, as opposed to more than eight or less than four. Yet the accuracy of the oft-touted view that late-night exercise disrupts sleep is far from apparent. It turns out that, almost universally, exercise completed within four hours of sleep increases total sleep time while still decreasing wake time after sleep onset. Since the evening is for many people the only feasible time to run or weight train, this is an important finding, and holds true across nearly all existing studies.
Good news for marathoners. Another interesting finding is that exercise duration significantly moderates sleep duration, with the greatest effects seen at exercise times over an hour. It appears that endurance runners may enjoy the most benefit in terms of longer, less interrupted sleep than other exercising populations or only occasional exercisers.
Intensity isn't an asset. It's worth noting that exercise need not be intense to reduce wake time after sleep onset -- light exercisers, in fact, reduce their time awake after turning in by about 16 minutes; high-intensity exercisers increase their time awake by approximately four minutes. A 2003 study of 173 overweight post-menopausal women found that even low-intensity stretching improved sleep quality and lengthened duration. And in 2004, researchers reported that three hour-long sessions of tai chi per week improved sleep quality in a group of elderly men and women.
Help for the elderly. In fact, older adults make up an important segment of the population for whom to explore insomnia cures. In a 2003 survey, the National Sleep Foundation found that 50 percent of adults over 50 reported having one or more symptoms of insomnia at least a few nights a week. This may in part be due to declining levels of the sleep-promoting hormone melatonin, poor bladder control, heartburn, diuretics such as Lasix or Bumex or use of the beta blockers Inderal and Lopressor.
It's encouraging, then, that one of the most promising areas of sound sleep promotion through exercise has been in studies of older adults. In a third study, of 43 adults over age 60 with moderate sleep complaints, a 16-week aerobic exercise regimen at 60 to 70 percent maximal capacity three to four times a week yielded an average sleep increase of 42 minutes versus controls. Self reported sleep quality was also significantly greater.
These studies illuminate one possible explanation for the present shortage of direct, compelling evidence that exercise promotes sleep: Most of the research thus far has been done on good sleepers, so study design may be imposing a ceiling on sleep improvement.
Controlled studies of individuals with insomnia or depression have shown significant effects in sleep promotion through exercise. The troubled sleepers in the study of elderly men and women above are where we should be looking for the greatest effects of exercise on sleep, and this is where they in fact appear. Unfortunately, most of these studies are limited by self-report measures. Further verification of these promising findings is necessary through objective sleep variables.
(Clin. Sports Med., 2005, Vol. 24, No. 2, pp. 355-365; Health After 50, 2004, Vol. 16, No. 10, pp. 4-5)
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