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Sleep is a naturally recurring state of mind and body, characterized by altered consciousness , relatively inhibited sensory activity, reduced muscle activity and inhibition of nearly all voluntary muscles during rapid eye movement REM sleep,  and reduced interactions with surroundings. Sleep occurs in repeating periods , in which the body alternates between two distinct modes: REM sleep and non-REM sleep. Although REM stands for "rapid eye movement", this mode of sleep has many other aspects, including virtual paralysis of the body. A well-known feature of sleep is the dream , an experience typically recounted in narrative form, which resembles waking life while in progress, but which usually can later be distinguished as fantasy.