There are two main types of sleep, REM (rapid eye movement) and non-REM (non-rapid eye movement). REM sleep occupies about 25% of our time while non-REM occupies about 75%. Each sleep cycle throughout the night lasts for about 90 minutes.

Once you are in non-REM sleep, there are four main stages. Stage one is often referred to as the "gateway to sleep" because it appears at the onset of sleep for a short duration. This is the shortest duration sleep, approximately 5% of the night. Stage two happens when the conscious awareness of the external environment is gone. This stage occupies about 45% of the night. Stages three and four are both referred to as "deep sleep". In these stages, brain activity is slowed and autonomic activity, heart rate, respiration, and blood pressured are all lowered. This is frequently the stage that people sleepwalk in. It is very difficult to wake someone up out of delta sleep and usually requires a 'personal' stimulus like calling out their name. These stages occupy about 25% of the night.