Most adults sleep between seven and eight hours a night. In a typical night, they go back and forth between two sleep phases. In the rapid-eye-movement phase (REM), the brain is highly active, the eyes move rapidly back and forth, and dreams occur frequently. In the non-REM phase (NREM), there is less brain activity and no eye movement. The deepest sleep, which occurs during the NREM period, is called slow-wave sleep.

This delicate cycle is unfortunately subject to disturbances. About one in ten people suffer from insomnia — defined as having trouble falling asleep, staying asleep, or waking up too early. Sleep disturbances are strongly related to mental illness. In a large American study, 40% of insomniacs also reported a psychiatric disorder, mainly anxiety and depression. Because exercise has been linked again and again with emotional benefits, it is possible that mental health gains associated with exercise may also improve sleep.

A few studies suggest that physical activity may be useful in preventing and treating sleep problems:

• In a 1985 survey of 1600 Finns, 33% of men and 30% of women rated exercise as the most important sleep-promoting factor. As many as 43% of those who increased their exercise over the previous three months reported improved sleep, whereas 30% of those who decreased their exercise over the same period reported worsened sleep. Sedentary respondents also reported a higher rate of excessive daytime tiredness-a symptom of inadequate sleep at night.
• In 1985, Trinder et al. reported that the type of physical training can influence both sleep onset and the amount of slow-wave sleep. Aerobic training was associated with an increase in slow-wave sleep, unlike power training or a combination of aerobic and power training, which led to decreases in slow-wave sleep.
• In a 1995 study, healthy but sedentary elders with normal sleep patterns participated in six months of either aerobic exercise or stretching and flexibility training. Both groups reported sleeping better at the end of the program. Only the aerobic exercise group showed a clear increase in slow-wave sleep, however.
• Physical activity may also play a role in consolidating sleep into a single period. Sleep and wakefulness patterns are clearly disrupted when individuals spend their days in complete idleness. Without activity, the prolonged sleep period of about eight hours ends up being replaced by short fragmented sleep bouts.

What type of exercise do we need to sleep better? Although there is no definite answer to this question at the present time, it appears that high-intensity exercise has the greatest effect on slow-wave sleep. However, sleep researchers are unsure about how changes in slow-wave sleep relate to changes in sleep quality. Until they come up with a precise answer, the best approach is to participate in aerobic activity, regularly.