I know that the number of hours you sleep is important. But is it also important which hours?

Dark hours. Circadian rhythms, which help define sleep schedule, are associated with light and dark. When we are exposed to light, our body releases a chemical that helps us stay awake. With training, the body can sleep at different hours and be awake at others. Generally getting enough rem sleep is important which happens about 90 minutes into a sleep session. However, you should try to keep to a schedule.