Sleep is broadly considered a biological necessity in virtually all animals. The large majority of such taxa with documented sleep physiology are bilaterians, though there is increasing evidence of sleep or sleep-like states in non-bilaterians such as Cassiopea jellyfish and hydra (both cnidarians), and sponges. The various criteria which biologists use to define sleep states have been observed in all other animal phyla, often with profound variation in function. In all of these taxa except sponges, regulation of sleep is documented to involve genes whose transcription oscillates with time, known as circadian or clock genes. These genes and the gene networks they regulate give rise to the internal circadian clock. The three categories of biological sleep schedules are diurnal, nocturnal, and crepuscular which characterize species whose waking periods mostly overlap with day, night, and twilight respectively. Specific sleep patterns and durations vary widely among and sometimes within sleeping species, with some sleeping or foregoing sleep for extended periods. Others, such as some porpoises, engage in unihemispheric sleep where only one brain hemisphere sleeps at a time in order to maintain motion or homeostasis in marine environments. The presence of some version of sleep physiology is nearly universally conserved within animals, suggesting a very ancient evolutionary origin. Chronobiological mechanisms more broadly (i.e. non-sleep biological processes which oscillate at various periods) are documented among many microbes and thus are likely much older, reflecting the day-night cycle as perhaps one of the earliest environmental stimuli used for the regulation of genes. This has led some evolutionary biologists to suggest that time-based oscillations in activity are a possible form of ancient, time-based niche partitioning to minimize competition during the same portions of the day.
Definition
Sleep can follow a physiological or behavioral definition. In the physiological sense, sleep is a state characterized by reversible unconsciousness, special brainwave patterns, sporadic eye movement, loss of muscle tone (possibly with some exceptions; see below regarding the sleep of birds and of aquatic mammals), and a compensatory increase following deprivation of the state, this last known as sleep homeostasis (i.e., the longer a waking state lasts, the greater the intensity and duration of the sleep state thereafter). In the behavioral sense, sleep is characterized by minimal movement, non-responsiveness to external stimuli (i.e. increased sensory threshold), the adoption of a typical posture, and the occupation of a sheltered site, all of which is usually repeated on a 24-hour basis. The physiological definition applies well to birds and mammals, but in other animals whose brains are not as complex, the behavioral definition is more often used. In very simple animals, behavioral definitions of sleep are the only ones possible, and even then the behavioral repertoire of the animal may not be extensive enough to allow distinction between sleep and wakefulness. Sleep is quickly reversible, as opposed to hibernation or coma, and sleep deprivation is followed by longer or deeper rebound sleep.
Necessity If sleep were not essential, one would expect to find
Animal species that do not sleep at all Animals that do not need recovery sleep after staying awake longer than usual Animals that suffer no serious consequences as a result of lack of sleep These symptoms are not seen in complex animals, and sleep is thus considered necessary to them. Sleep helps the body and mind to feel rested. Findings show that if rats do not get any sleep, they die in a few weeks. Despite having enough food, their appetite tends to decrease resulting in weight loss and eventually death. Outside of a few basal animals that have no brain or a very simple one, no animals have been found to date that satisfy any of these criteria. While some varieties of shark, such as great whites and hammerheads, must remain in motion at all times to move oxygenated water over their gills, it is possible they still sleep one cerebral hemisphere at a time as marine mammals do. However, it remains to be shown definitively whether any fish is capable of unihemispheric sleep.
Invertebrates
The fresh-water polyp Hydra vulgaris and the jellyfish Cassiopea are among the most primitive organisms in which sleep-like states have been observed. Observing sleep states in jellyfish provides evidence that sleep states do not require that an animal have a brain or central nervous system. The nematode C. elegans is another primitive organism that appears to require sleep. Here, a lethargus phase occurs in short periods preceding each moult, a fact which may indicate that sleep primitively is connected to developmental processes. The results of Raizen et al. furthermore suggest that sleep is necessary for changes in the neural system.
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