Sleep is a recurring state of relaxation that is characterized by an altered state of consciousness, inhibited sensory activity, muscular inhibition, and severely reduced interaction with outside entities. This state alternates between REM (rapid eye movement) sleep, during which dreams occur, and non-REM sleep. While in REM sleep the mind dreams and the body experiences a state of near paralysis that keeps a person from physically acting on dream stimulus (this is called sleep paralysis). Sleep itself is a state in which the body builds up and restores the immune, nervous, skeletal, and muscular systems. The purpose of dreaming is still not clearly understood but appears to be a time and method of processing mental and emotional input.