Late-day exercise: Can it cause insomnia?

Several studies indicate that regular exercise improves sleep in people with insomnia. But how the timing of exercise affects your ability to fall asleep is unclear. Here's what the research indicates:

• A study of postmenopausal women showed that those who exercised in the morning had a better quality of sleep than did those who exercised at night.
• A study of highly active young men found that prolonged, vigorous exercise 30 minutes before bedtime had little effect on sleep.

Most people with chronic insomnia have increased levels of stress hormones in their blood, suggesting that they are abnormally sensitive to stress (hyperaroused). The higher your stress hormone levels, the worse your sleep is. Exercise initially increases such stress hormones. But several hours after a workout, a rebound effect occurs and stress hormones decrease to a level that's lower than if you hadn't exercised.

The time it takes for stress hormones to decrease following exercise varies, depending on the individual and the intensity of the activity. In general, it takes four to six hours after a high-intensity 20-minute aerobic workout before stress hormones decrease.

Because each person is different, you may consider trying this experiment if you're having trouble falling asleep:

• For two weeks, exercise close to bedtime. Each morning, rate your quality of sleep on a scale of 1 to 10.
• For the next two weeks, exercise in the morning or in the early afternoon. Each morning, rate your sleep quality.
• Average your rate for each of the two weeks.

This can show you how the timing of your exercise may affect your sleep and help you determine the time of day that's best for you to exercise.