For the study, researchers looked at whether low, moderate, and vigorous sessions within four hours of bedtime affected sleep. They found that people who logged a high-intensity routine within one hour of bedtime took 14 minutes longer to fall asleep than normal.
Your body’s parasympathetic activity increases as your bedtime approaches to help you fall asleep, says lead study author Christina Spengler, Ph.D., deputy head of the Institute of Human Movement Sciences and Sport at ETH Zurich in Switzerland. But her team’s analysis found that if you hit it hard at night, your heart rate will remain elevated for about an hour, which can keep your body from switching to rest mode.
Wrap up HIIT training at least two hours before turning in, Spengler says. If you’re exercising later on, opt for a less intense routine that keeps your heart rate below 76 percent of your max.