We sleep cycles which are approximately 90 minutes in length. In a typical night of 7-8 hours of sleep, an individual is likely experiencing 4-5 complete cycles. Within each cycle, there are distinct stages:
rapid-eye-movement sleep (REM sleep) or
non-rapid-eye-movement sleep (NREM sleep).
Usually, sleeper begins sleep in NREM stage 1, then spends some time in each consecutive NREM stage before jumping to REM for a time and then restarting the cycle NREM sleep is further broken down into either three or four separate cycles depending on which sleep scientist you ask (most sleep scientists decided that stage 3 and stage 4 sleep were essentially the same thing).