Many people think that sleep is simply a time when the body switches off, but scientists know that the brain stays very active at night. During sleep, the brain moves through several stages, including a stage called REM, when most dreams happen. Sleep helps us remember what we learned during the day, and it also gives the body time to repair itself. Most adults need between seven and nine hours each night, while young children need much more. When people do not sleep enough, they find it hard to concentrate, and they may feel angry or sad more easily. Doctors suggest a few simple habits for better sleep. Go to bed at the same time every day, avoid strong coffee in the evening, and keep your bedroom quiet and dark. Using a phone in bed can also be a problem, because the bright screen may make it harder to fall asleep.

Question: In the sentence 'it also gives the body time to repair itself', what does 'it' refer to?