Virtually everyone has at least one overachieving friend or acquaintance who finishes an intensive workout each morning before sunrise. On the contrary, those who prefer to exercise just before bed are few and far between. Average Janes and Joes seem to fall somewhere in the middle, choosing to sweat it out later on in the morning, midday, or early evening. Of course, plenty of people prefer to not exercise at all.

Regardless of the type of workout, fitness enthusiasts have long wondered the optimal time of day for maximum results during and after intense physical activity. Among other suspicions, many posit that exercising at night can impede sleep, or at least delay it.

New research findings suggest that the concept may be a myth, according to a study from the Institute of Human Movement Sciences and Sport at ETH Zurich, which was posted on Science Daily last month.

Author Jan Stutz summarized the findings, writing:

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. 

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.

There are other factors to consider in making a decision regarding what time is best for a specific individual to exercise. However, most often those variables are relative to the person’s circumstances. For most, life priorities and demands tend to influence the effectiveness of a workout more than the earth’s rotation each day.

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