Thinking you worked out a lot improves sleep. How hard you actually worked had no effect.
Via Dreamland: Adventures in the Strange Science of Sleep:
What may matter more than the amount of time spent doing strenuous physical activity is how hard the brain considers the work to be. One study, completed by Swiss researchers and published in the Journal of the American College of Sports Medicine, focused on nearly nine hundred college students in Switzerland. Each person in the study tracked how much he or she exercised during the week. Subjects were also asked to fill out two questionnaires. In one, they rated how well they slept, on a scale of 1 to 10. In the other, they rated their overall fitness level, again on a scale of 1 to 10. Overall, the research team found no link between how many hours a student spent working out and how well he or she slept each night. The self-assessments uncovered other surprising results, however. Nearly a fifth of those who rated themselves low on the fitness scale were, in reality, among the most physically active participants in the entire study group. These subjects were working out all the time, but they simply felt like they weren’t doing enough. Those perceptions carried over into their sleep… The study found no relationship between an individual activity level during the day and the quality of sleep that night, which means that a long run alone won’t provide the answer to a better night’s sleep.
Intriguingly, the subjects who thought they were in good shape slept well— even if they weren’t actually exercising as much as others in the study. It appeared that whatever amount of time these subjects spent working out was enough to cross the mental threshold that told them not to worry about their fitness level. As they laid down each night, their lack of concern about whether they were exercising enough gave them one less thing that could stand in the way of drifting off to sleep. Their minds thought their bodies were meeting the standard, and they acted accordingly. As the lead researcher for the Swiss study told the New York Times, “What people think is more important than what they do.”
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