Most of us think that exercise improves sleep. But it may be that thinking that exercise improves sleep improves sleep. That, at any rate, is the provocative finding of a new study completed recently in Switzerland and published last month in the journal of the American College of Sports Medicine.
For the study, 862 Swiss college students were asked to record how much they exercised, how fit they believed themselves to be (on a scale from 1 to 10) and how well they slept (on a scale from 1 to 8). The correlations between how much some of the students exercised and how fit they believed themselves to be was not very precise. More than 16 percent of the students who rated themselves low on the fitness scale actually exercised the most. In other words, they worked out more than many of the other students but felt they weren’t doing enough.
Those students who perceived that they weren’t exercising enough also tended to report sleeping less well, even though they were exercising more than some of the other students. In the end, the researchers found almost no correlation between how much students exercised and how well they slept. What mattered was whether they believed that they were being active enough. Those students who perceived that they were fit slept well. Those who didn’t, did not.
As Markus Gerber, a researcher at the Institute of Exercise and Health Sciences at the University of Basel and lead author of the study told me in an e-mail message that the findings suggest that, when it comes to the role of exercise and sleep, “what people think is more important than what they do.”
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That conclusion is not, in fact, anomalous. It actually fits neatly into a large if little-known body of science intimating that exercise may not ease you to sleep after all. The relationship between exercise and sleep “is certainly complicated,” said Shawn Youngstedt, an associate professor of exercise science at the University of South Carolina and author of some of the science in question.
In a representative study that he led several years ago, for instance, college students — some athletic, some sedentary — kept detailed sleep and exercise diaries for months. At the end of that time, the researchers cross-referenced the diaries and found no notable correlation between exercising more and sleeping better or vice versa. Meanwhile, in a second part of the same study, a group of adults wore monitors that recorded their movements and sleep patterns. The participants also filled out activity diaries. Using the objective data from the monitors, together with the diary reports, the researchers found only marginal impacts on sleep from exercise. The most active volunteers tended to fall asleep about a minute and a half faster than those who were the least active. Otherwise, their sleep was virtually identical.
But most of us continue to believe that working out is helpful to snoozing, as shown in a number of large-scale, international surveys completed in the past decade, during which respondents have consistently reported that they slept better on days when they exercised. Parse the results closely, though, and the link grows more tenuous. Respondents often report, for instance, that they sleep well on the weekends, when they have enough time to exercise, but also when they’re free of bosses, deadlines, rushed lunch breaks and other stressors that can disrupt sleep. Similarly, as Mr. Youngstedt points out, the surveys rarely are able to take into account such confounding issues as smoking, weight, anxiety or exposure to sunlight, all of which have been found in studies to affect sleep.
Perhaps the most surprising finding of the current science about exercise and sleep, however, is that wearing yourself out physically is not the same as being sleepy. “The two are easily mistaken,” Mr. Youngstedt said, but they seem to affect various bodily systems at the same time in different ways. Hard, long workouts or severe overtraining may be just as likely, in fact, to lead to wakefulness and sleep problems, Mr. Youngstedt said, than to better sleep. On the other hand, a related truism about exercise and sleep appears to be a myth. There is “absolutely no scientific evidence” that working out in the late evening keeps you from sleeping, Mr. Youngstedt said.
In the end, the current state of the science about sleep and exercise is somewhat cloudy. But more clarity may come, Mr. Youngstedt said. He, for one, is not convinced that there is no link between exercise and sleep. But most laboratory studies to date, including his, have involved volunteers without underlying sleep problems.
“There’s no room for exercise to improve sleep, if people are sleeping fine,” he said.
To sidestep that issue, he and his colleagues have a number of studies under way that use volunteers with sleep pathologies, like sleep apnea, to discern whether exercise helps in those cases. Results, so far, Mr. Youngstedt said, “are promising.”
Still, the most practical advice that science can offer at the moment about exercise and sleep is not to fret too much about whether you’re getting enough of either. Worrying, as the Swiss study showed, is what will keep you awake long into the night.
“I would give the following recommendation,” Mr. Gerber, the Swiss scientist, wrote. “It does not matter how much exercise” you actually complete “as long as it make you feel good and feel fit.”