Scientists divide sleep into two major types: non-REM or quiet sleep and REM (rapid eye movement) sleep. They are as different from one another as sleeping is from waking.
Quiet Sleep: This type of sleep has been called "an idling brain in a moveable body" and is made-up of 4 stages. Thinking and most physiological activities slow down, but movement still occurs. Once the eyes are closed and nerve cells no longer receive visual input, brain waves settle into a steady and rhythmic pattern, which is characteristic of calm, relaxed wakefulness.
Stage 1 - As people transition from wakefulness into light sleep, they spend about 5 minues in Stage 1 sleep. During Stage 1, body temperature drops, muscles become relaxed and eyes move from side to side. People lose awareness of their surroundings, but can be easily jarred into wakefulness.
Stage 2 - This is the first stage of true sleep and lasts 30-45 minutes per episode. The eyes are still and the heart rate and breathing are slower than when awake. About half the night is usually spent in Stage 2 sleep, which leaves peoples feeling moderately refreshed.
Stage 2 and 4 - Together these are known as deep sleep. Breathing becomes more regular, blood pressure falls and pulse rates slow until 20-30% below waking rates. The brain becomes less responsive to external stimuli, making it difficult to wake the sleeper. Deep sleep is a time for the body to renew and repair itself.
Dreaming Sleep: REM sleep has been called "a hyperactive brain in a paralyzed body." The brain races, thinking and dreaming as the eyes dart back and forth rapidly behind closed eye lids. Muscles not needed for breathing and eye movement are temporarily paralyzed. About 3-5 times per night (about every 90 minutes) a sleeper enters REM sleep for several minutes. Altogether, REM sleep makes up about 20% of total sleep.