Q. Will exercising at night make it hard to sleep?

A. With our busy lifestyles of working all day, then attending meetings or helping the kids with homework, you may not have time to squeeze in exercise until later in the evening. The good news is that for most people, it doesn't matter what time of day you work out. The most important thing is that you just do it!

For years, people have been advised to avoid exercise for at least three hours before bedtime. The rationale is that it takes several hours for stimulant hormones that surge during a workout and body temperature to return to normal levels.

Although this sounds logical, there is little scientific evidence to back it up. Shawn Youngstead, of the University of South Carolina, has done extensive research on sleep and exercise. He has found no positive or negative effect from exercising before bed.

In one study he conducted, men who exercised vigorously as late as half an hour before bedtime experienced an elevation in body temperature, but had no trouble going to sleep. The hike in body temperature may actually help us sleep because the area of the brain involved in lowering body temperature is also involved in promoting sleep.

In a study published in the journal Physiology and Behavior, a group of college students exercised moderately for an hour on two separate nights, in one case 90 minutes before bedtime, and in the other 30 minutes before bedtime. The activity had no impact on the amount of time the subjects needed to fall asleep or their number of waking episodes during the night.

There certainly are some individuals who cannot fall asleep after exercising. People who feel exercise disrupts their sleep should alter their workout schedule and see if it makes a difference. Consistency is also important. If you exercise about the same time most evenings, your body's internal clock will adjust.

The best time to exercise is the time that is right for you. Morning workouts help some people rev up their body to start the day, releasing endorphins and enhancing their mood. Others find afternoon or evening workouts productive. Our muscles are warm and flexible in the evening.

Evening workouts can help us decompress and relieve stress after a day of constant mental stimulation. This makes it easier to relax and fall asleep.

Amy Tousman is a registered dietitian and certified diabetes educator with Nutrition Unlimited in Kailua. Hawai'i experts in traditional medicine, naturopathic medicine and diet take turns writing the Prescriptions column. Send your questions to: Prescriptions, Island Life, The Advertiser, P.O. Box 3110, Honolulu, HI 96802; islandlife@honoluluadvertiser.com; or fax to 535-8170. This column is not intended to provide medical advice.