Sleep in animals

Sleep is a recurring, reversible state in animals marked by reduced responsiveness to the environment. Its timing and intensity are regulated by circadian rhythms and accumulated sleep pressure.

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Sleep homeostasis: The process by which sleep pressure builds during wakefulness and declines during sleep. It explains why prolonged wakefulness increases subsequent sleep duration or intensity.
Sleep state: A recurring physiological condition with characteristic patterns of responsiveness, brain activity, and bodily regulation. Animal sleep is one form of this broader biological state.
Dolphin sleep: A sleep pattern in dolphins in which one brain hemisphere sleeps at a time. It illustrates how aquatic mammals maintain breathing and environmental awareness during sleep.
Torpor: A regulated state of reduced metabolism and body temperature used by some animals to conserve energy. Unlike ordinary sleep, torpor involves pronounced metabolic suppression.
Circadian rhythm: An approximately 24-hour biological cycle synchronized with environmental cues such as light. It determines when sleep propensity rises across the day-night cycle.
Behavioral state: A coordinated pattern of an animal’s activity and responsiveness over a period of time. Sleep is classified partly by observable changes in behavior.
Bird sleep: Sleep in birds, including non-REM and REM-like stages and species-specific adaptations. Some birds sleep with one hemisphere at a time, especially in exposed settings.
Hibernation: A prolonged seasonal state involving reduced metabolism, body temperature, and activity in some animals. It lasts far longer than typical sleep and includes substantial physiological changes.
Melatonin: A hormone that signals biological night in many animals and helps regulate circadian timing. Its daily secretion links darkness to shifts in sleep timing.
Sleep deprivation: A reduction or prevention of sleep that can produce compensatory sleep and physiological effects. Rebound after deprivation helps identify sleep’s homeostatic regulation.