Exercise is often associated positively with a good night's sleep. But if done at certain times of day, or shortly before bedtime, it can also alter how we sleep. A new meta-analysis by Concordia researchers published in the journal Sleep Medicine Reviews assessed data from 15 published studies to see how a single session of intense exercise affects young and middle-aged healthy adults in the hours prior to bedtime. The researchers found that the combination of factors would interact to enhance or modulate the effects of exercise on sleep. Timing is (almost) everything. Emmanuel Frimpong, a postdoctoral fellow at the Sleep, Cognition and Neuroimaging Lab and the study's lead author, says their principal goal was to assess whether high-intensity exercise affected sleep afterwards and to see which factors might influence that sleep. The researchers combined data from the 15 studies and ran a statistical analysis that examined variables such as the timing of exercise -- early evening or late evening -- and the hours between the cessation of exercise and bedtime -- less than two hours, around two hours, and two to four hours. Overall, our analysis showed that when exercise ended two hours before bedtime, there were sleep benefits, including the promotion of sleep onset and increased sleep duration. On the other hand, when exercise ended less than two hours before bedtime, sleep was negatively impacted. It took longer for participants to fall asleep and sleep duration decreased. Based on our review, for healthy, young and middle-aged adults with no history of sleep disorders, evening exercises should be performed in the early evening if possible. Individuals should also keep to a consistent exercise schedule, as exercising at different times of the evening could cause sleep disturbances. Individuals should also consider whether they are morning people or evening people. High intensity exercise performed late in the evening can result in sleep disturbance for morning-type people. Sleep hygiene strategies should also be carried out, such as taking a shower between the cessation of exercise and bedtime and avoiding eating heavy meals or drinking a lot of water before going to bed.