Sleep is a dynamic process with a complex 'architecture' of its own. Sleep is divided into 2 stages:

REM stands for (R(apid) E(ye) M(ovement).

The Non-REM sleep is characterizated by a slow decrease in brainwaves, the body gradually relaxes. This
period is divided into 4 stages:

Stages 3 and 4 of non-REM sleep are most important for the body. It allows the brain to go on a little vacation needed to restore the energy we expend during our waking hours. Blood flow decreases to the brain in this stage, and redirects itself towards the muscles, restoring physical energy. Research also shows that immune functions increase during deep sleep.

REM-Sleep:
In this phase breathing becomes more rapid, irregular and shallow, eyes jerk rapidly and limb muscles are temporarily paralyzed. Brain waves during this stage increase to levels when a person is awake. Also, heart rate increases, blood pressure rises and the body loses some of the ability to regulate its temperature. This is the time when most dreams occur.

Sleep stages:
Normally someone falls asleep within 10 to 15 minutes after going to bed. You begin your nightly journey by descending into stage 1, a light sleep. Your muscles relax, your brain waves are irregular and rapid. In stage 2, brain waves become larger, with bursts of electrical activity. Then you move into deep sleep (stages 3 and 4 of the non-REM sleep), in which the brain produces large, slow waves.

After an hour or so, you shift into a highly active stage and the first REM-sleep occurs. Suddenly your brain waves are almost the same as if you were awake. You’re in the dreaming stage. Dreams vary in length according to the length of REM-sleep. After the REM-sleep the deep sleep (stages 3 and 4 of the Non-REM sleep) starts again. The different sleep stages follow each other, in a certain variety, with a certain regularity during a full night sleep. One non-REM sleep and one REM-sleep is called a cycle. During the night several cycles occur, about once every 90 minutes in humans.

About 75% of your night is spent in non-REM sleep and about 25% in REM-sleep. REM periods tend to become longer and more plentiful as the night wears on.