What does exercising right before bed do to my sleep?

Evening exercise does not harm your sleep as a general rule, but the timing and intensity make a big difference. A systematic review and meta-analysis of 23 studies shows that exercising in the evening slightly increases deep sleep (by 1.3 percentage points) and reduces light sleep. Those are favourable signs.

Things change when you do very intense training close to bedtime. That same research found a negative effect on sleep-onset time and sleep quality with very vigorous exercise within one hour of bedtime. That effect was statistically fragile: it disappeared as soon as one study was removed from the analysis. Even so, it fits into a broader picture. Observational research covering nearly 4 million measured nights in athletes shows that the later and harder the workout, the later you fall asleep, the less you sleep, and the higher your heart rate remains during the night. These are signs of a nervous system that is still in full active mode.

A concrete threshold: when a workout was completed at least 4 hours before bedtime, that same large observational study found no adverse effects on sleep or nocturnal heart function, regardless of how hard the exercise had been. How far before bedtime you exercise therefore appears to matter more than whether you train in the evening or the morning.

Body temperature plays a role here. A higher temperature at bedtime, caused by exercise, was associated with sleep efficiency that was on average nearly 12 percentage points lower, and on average 38 minutes more time awake after falling asleep per degree of higher temperature. Cooling down after exercise is therefore far from a luxury.

Additional nuance applies to people who already sleep poorly. A study in older adults with difficulty falling asleep found that morning exercise improved objective sleep quality, whereas evening exercise did not provide that benefit. In the long run, regular exercise helps in all cases: a meta-analysis of 66 studies found shorter sleep-onset time, more total sleep time, and better sleep quality in people who trained consistently over several weeks.