Sleep is broadly classified into two types: REM (rapid-eye-movement) sleep and non-REM sleep (NREM).

In NREM sleep your muscles are more relaxed than when you are awake but you are able to move (although you do not because the brain is not sending signals to the muscles to move).

REM sleep is more active. Breathing and heart rate become irregular, your eyes move rapidly back and forth under your eyelids, and control of your body temperature is impaired so that you do not sweat when you are hot or shiver when you are cold.

Below the neck, however, you are inactive because the nerve impulses that travel down the spinal cord to body muscles are blocked.

Your body is essentially paralyzed. It is during this sleep stage that you dream.

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