The Stages of Sleep

Stage 1 straddles the line between alert and asleep.

Stage 2 is when your body temp cools and you're oblivious to your surroundings.

Stages 3 and 4 send you into deeper levels of sleep. Your breathing slows, your blood pressure drops, and your muscles relax.

You then move into rapid eye movement (REM) sleep for anywhere from five to 40 minutes at a time. "In REM sleep, the brain is awake but you're unaware of your surroundings, unconscious in one sense but totally conscious in another," explains Philip Westbrook, M.D. The stage occurs every 90 minutes throughout the night, and it's during REM that you have your most vivid dreams.

Once you've snoozed your way through each level, you typically go from REM right back into stage two. It all tapers off in the a.m., when your body releases cortisol to help you shrug off sleepiness as you open your eyes.

Source:health.yahoo.net