Understanding Sleep.

Sleep is usually classified into two: orthodox sleep and paradoxical sleep. Orthodox sleep is also called non REM or NREM (Non Rapid Eye Movement) sleep and paradoxical sleep is often called the REM sleep.

NREM sleep has four different stages:
in the first stage, blood supply to the brain is reduced and the temperature in the brain falls slightly. Heart beat and breathing are accelerated. The muscles are slightly tensed. Thoughts cris-cross the mind.
In the next stage, unconnected thoughts dominate the mind. Slowly the sleeper moves to the third stage in which the heart rate and breathing become slow. The muscles of the body becomes relaxed. The brain starts to emit delta waves.
In the next stage, we have the deep sleep which is full delta wave sleep. The orthodox sleep is broken up by about five periods of REM sleeps each of which is longer duration than the previous one.
In the REM sleep, the sleeper breaths irregularly. The blood supply to the brain is increased and so is the temperature in the brain. Most of the muscles are relaxed in this stage. Jerky movements of body and face are characteristic of this stage of sleep. An average person spends about 25% of his sleep in REM sleep. New born children have REM sleep: about half of their sleep is REM sleep.
During stages 3 and 4, growth hormones are secreted into the blood stream. Body's manufacture of proteins, sexual arousal, orgasm etc. are also significantly influenced by REM sleep. Earlier, sleep researchers believed that REM sleep is essential for our mental health. But this is not accepted now.