Sleep is a naturally recurring state of mind and body, characterized by altered consciousness, reduced perception of external stimuli, and inhibited voluntary muscle activity. It is distinguished from wakefulness by a decreased ability to react to stimuli, as well as a reduction in the amount and quality of thoughts that a person has. A person during sleep is generally unaware of their surroundings. Sleep occurs in repeating cycles during which different stages are experienced. The most common stages are deep sleep (slow-wave sleep) and rapid eye movement (REM) sleep. Dreams usually occur during REM sleep. Sleep deprivation can adversely affect mental health and physical health.