When you first start falling asleep you enter a transitional state for half a minute to seven minutes. At that time you are still aware of the environment, thoughts drift, and the body temperature starts dropping, sometimes you feel like you jerk in order to save yourself. Your eye movements at this stage are slow and rolling. The sleeper has no eye movements and short thoughts. State three is when you are in deep sleep-Delta sleep.
Delta sleep is more prevalent in the first third of the night.
The growth hormones are active at this stage. The fourth stage is when physical restoration and healing take place. A person who is deprived of stages three and four becomes apathetic and depressed. This could also lead to fibrositis. REM sleep starts after about 90 minutes of sleep.