About Advice > Sleep disorder

What happens when you sleep?

Many people think of sleep as a passive activity, but sleep is actually an active state. It restores us, helps the body to repair damage and grow new cells, keeps the body's nervous system working properly, and helps us to consolidate memory (helps us to remember what we learned during the day). During sleep, a person passes through 5 phases, or stages, of sleep - stages 1, 2, 3, 4 of quiet sleep and stage 5, called REM (rapid eye movement) sleep.

• Stage 1 sleep is light sleep, where we drift in and out of sleep and can be woken up easily. Eyes move very slowly and muscle activity slows down.
  
  
• A person spends almost half of their total sleep time in Stage 2 sleep. Eye movements stop and brain waves (or activity) become slower.
  
  
• Stages 3 and 4 are called deep sleep. During Stage 3 sleep, brain waves slow down even more and the brain makes mostly delta waves (slow brain waves). The brain makes only delta waves during Stage 4 sleep and there is no eye movement or muscle activity. People often feel groggy and disoriented for a few minutes when they are woken up during deep sleep. Some children have bedwetting, night terrors, or sleepwalking during deep sleep. Deep sleep restores us, helping to grow new cells and repair cells from damage.
  
  
• A person's breathing becomes more rapid, irregular, and shallow in REM sleep. The eyes jerk quickly in many directions, heart rate increases, and blood pressure rises. When people wake up during REM sleep, they often describe strange dreams that don't make any sense. Most dreaming happens during REM sleep. REM sleep is important, perhaps in part because it stimulates the parts of the brain that help us learn.

A person cycles through these 5 stages of sleep during the night. The first sleep cycles contain short REM periods and long periods of deep sleep. REM sleep periods become longer in length while deep sleep decreases. By morning, almost all sleep time is in stages 1, 2, and REM.