Sleep

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"Waking up" redirects here. For other uses, see Waking Up (disambiguation).

This article is about sleep in general; for specifically non-human sleep see Sleep (non-human); for other uses, see Sleep (disambiguation).

Sleeping child

Sleep is a naturally recurring state characterized by reduced or absent consciousness, relatively suspended sensory activity, and inactivity of nearly all voluntary muscles.^[1] It is distinguished from quiet wakefulness by a decreased ability to react to stimuli, and is more easily reversible than being in hibernation or a coma. Sleep is a heightened anabolic state, accentuating the growth and rejuvenation of the immune, nervous, skeletal and muscular systems. It is observed in all mammals, all birds, and many reptiles, amphibians, and fish.
The purposes and mechanisms of sleep are only partially clear and are the subject of intense research.^[2] Sleep is often thought to help conserve energy,^[3][4] but actually decreases metabolism only about 5–10%.^[3][4] Hibernating animals need to sleep despite the hypometabolism seen in hibernation, and in fact they must return from hypothermia to euthermia in order to sleep, making sleeping "energetically expensive."^[5]

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laughter

Laughing is a reaction to certain stimuli. Traditionally, it is considered a visual expression of happiness, or an inward feeling of joy.^{[citation needed]} It may ensue from hearing a joke, being tickled, or other stimuli.
Laughter is a part of human behavior regulated by the brain, helping humans clarify their intentions in social interaction^{[citation needed]} and providing an emotional context to conversations.^{[citation needed]} Laughter is used as a signal for being part of a group — it signals acceptance and positive interactions with others.^{[citation needed]} Laughter is sometimes seen as contagious, and the laughter of one person can itself provoke laughter from others as a positive feedback.^[1] This may account in part for the popularity of laugh tracks in situation comedy television shows.^{[citation needed]}
Laughter is anatomically caused by the epiglottis constricting the larynx.^{[citation needed]}
The study of humor and laughter, and its psychological and physiological effects on the human body, is called gelotology

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