Even among sleep researchers, it is a widely held belief that sleep quality can be improved by avoiding exercise in the evening. However, as researchers from the Institute of Human Movement Sciences and Sport at ETH Zurich have demonstrated, it is not generally true.

The scientists combed through the literature on the subject and analysed all 23 studies that met their quality requirements. They concluded that doing exercise in the four hours before going to bed does not have a negative effect on sleep. “If doing sport in the evening has any effect on sleep quality at all, it’s rather a positive effect, albeit only a mild one,” says Christina Spengler, head of the Exercise Physiology Lab at ETH Zurich.

By combining the data from the different studies, the researchers showed that in the night after study participants had done some sport in the evening, they spent 21.2 percent of their sleeping time in deep sleep. Following an evening without exercise, the average figure was 19.9 percent. While the difference is small, it is statistically significant. Deep sleep phases are especially important for physical recovery.

An exception to the rule

Vigorous training within an hour before bedtime is an exception to the rule. According to this analysis, it is the only type of evening exercise that may have a negative effect on sleep. “However, this preliminary observation is based on just one study,” Spengler says.

“As a rule of thumb, vigorous training is defined as training in which a person is unable to talk. Moderate training is physical activity of an intensity high enough that a person would no longer be able to sing, but they could speak,” Spengler says. One example of vigorous training is the kind of high-intensity interval training that competitive athletes often perform. In many cases, though, a longer endurance run or a longer ride on a racing bike would fall into the moderate training category.

As the analysis showed, it took study participants who completed an intensive training session shortly before bedtime longer to fall asleep. The study also provided insight into why this is the case: the test subjects were not able to recover sufficiently in the hour before they went to bed. Their hearts were still beating more than 20 beats per minute faster than their resting heart rate.