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In 2013, the U.S. National Sleep Foundation released the results of a 1,000-person poll that largely confirmed common sense: Good exercise, it found, is associated with good sleep.Edward J Bock III/Getty Images/iStockphoto

This is the ninth and final story in a print and online series looking at the science of sleep and the vital role of sleep in maintaining overall health.

It's human intuition: When you're wired, try going for a bike ride to tucker yourself out before bedtime. If you want your high-energy kids to crash, send them outside to run around for a while. Burning more calories, generally, means counting fewer sheep – the relationship between sleep and exercise, for most people, boils down to common sense.

Trouble is, one person's intuition doesn't apply to everyone. Things get trickier when the science comes in. "It's long been assumed and anecdotally claimed that exercise improves sleep," says Christopher Kline, an assistant professor of health and physical activity at the University of Pittsburgh. The problem, he says, is that until a few years ago, much of the research focused on people who already slept well.

In 2013, the U.S. National Sleep Foundation released the results of a 1,000-person poll that largely confirmed common sense: Good exercise, it found, is associated with good sleep. But Dr. Kline, one of the lead researchers in that study, says it's just the beginning. "You'd be surprised at how little quality evidence there is in terms of exercise improving sleep in people who actually have poor sleep," he says.

Bit by bit, though, researchers are starting to unfold how exercise can help – or hinder – specific groups of problem sleepers, from insomniacs to spread-thin working mothers. Dr. Kline, for instance, is doing pioneering research into the relationship between physical activity and sleep-apnea symptoms. Sleep and exercise certainly seem to go hand-in-hand, but research is now finally probing their relationship deeper, and among groups of people.

More and better research, Dr. Kline says, could help the population become healthier. "There's still a long way to go before sleep can be regarded as important as physical health behaviours and diet," he says.

The 2013 NSF study found that most exercisers were likely to report having a good night's sleep during the week, with vigorous exercisers declaring great sleep twice as frequently as non-exercisers. Because the results were self-reported, the researchers hesitated to call it cause-and-effect, but called the relationship "compelling."

"We can pretty confidently say exercise improves sleep," Dr. Kline says, "but in terms of translating that to a public-health message to recommend something – we can't really make any substantive recommendations based on the research."

Dr. Kline is one of a growing number of scientists doing work to change that. His recent work includes a randomized, controlled study of 43 overweight and obese adults with untreated obstructive sleep apnea – a disorder that causes pauses in breathing during sleep. The study found that, with exercise training, sleep apnea symptoms can be reduced. He has also done a meta-analysis in the journal Lung of five exercise-and-sleep-apnea studies that found similar results, including an indication that exercise can reduce symptoms of the disorder even if it doesn't result in weight loss.

A lot of the randomized, controlled studies related to sleep and exercise have had relatively small numbers of participants, Dr. Kline says. "In other fields of research, definitive randomized trials would have hundreds, or thousands, of participants, and rigorous experimental design. We're just basically continually pushing out pilot studies."

Those studies, at least, are starting to pile up, examining not just specific diseases but also differing demographics. Studies have been released this year, for instance, that found exercise training can improve sleep for both seniors and obese adolescents.

University of Ottawa professor Michelle Fortier, meanwhile, published a paper in Mental Health and Physical Activity a few months ago that measured both physical activity and sleep satisfaction in working mothers to assess their effects on mood. While the study found that, among the 63 participants, exercise didn't improve sleep satisfaction, both factors were found to predict positive moods, with exercise pulling ahead.

"What we found with them might not translate to the vast majority of the population," Dr. Fortier says of the high-intensity sample group. "They weren't just taking a couple of walks a week." This, she says, opens up a great opportunity for future research on high-intensity people. "Some people sacrifice their sleep to be able to get it all done," she says. "It'd be interesting to look at it from that perspective."

There is certainly no shortage of angles from which sleep and exercise can be examined. At Calgary's Centre for Sleep & Human Performance, Charles Samuels is looking at their relationship in the inverse: that is, how sleep can enhance physical performance, particularly among athletes. His team has developed athlete sleep screening tests and jet-lag management programs.

But at his lab, Dr. Samuels sees patients of all stripes, and always encourages them to try exercise to improve sleep. "More activity is better," he says. "When it's all said and done, it's generally going to be better for your sleep."

As for deeper research – into the mechanisms, the demographic differences, the public health implications behind that intuitive suggestion? Well, it's still early days. "The fact is," Dr. Samuels says, "we don't know a lot."

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