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Researchers have looked to animals exhibiting USWS to determine if sleep must be essential; otherwise, species exhibiting USWS would have eliminated the behaviour altogether through evolution. [3] The amount of time spent sleeping during the unihemispheric slow-wave stage is considerably less than the bilateral slow-wave sleep.
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 ...
To adapt to predation, two common techniques have evolved: positioning oneself out of harm's way while sleeping, and sleeping more lightly (such as unihemispheric sleep). In birds, perch height is believed to play a significant role in sleep; lower perch height has been shown to reduce the number and length of REM sleep episodes in pigeons, and ...
Slow-wave sleep is necessary for survival. Some animals, such as dolphins and birds, can sleep with only one hemisphere of the brain, leaving the other hemisphere awake to carry out normal functions and to remain alert. This kind of sleep is called unihemispheric slow-wave sleep, and is also partially observable in humans. Indeed, a study ...
The only common observation is that reptiles do not have REM sleep. [7] Sleep in some invertebrates has also been extensively studied, e.g., sleep in fruitflies (Drosophila) [40] and honeybees. [41] Some of the mechanisms of sleep in these animals have been discovered while others remain quite obscure.
Sperm whales have the largest brain mass of any animal on Earth, averaging 8,000 cm 3 (490 in 3) and 7.8 kg (17 lb) in mature males. [28] The brain to body mass ratio in some odontocetes, such as belugas and narwhals, is second only to humans. [29] In some whales, however, it is less than half that of humans: 0.9% versus 2.1%. [citation needed]
The researchers who documented this behaviour called it "sleep-swimming". [35] Sleep could also be absent during specific parts of a fish's life. Species normally quiescent at night become active day and night during the spawning season. [1] Many parental species forego sleep at night and fan their eggs day and night for many days in a row.
A correlation exists between body size and sleep pattern in duikers. While smaller to medium-sized duikers show increased activity and scavenge for food during the daytime, larger duikers are most active at night. [14] An exception to this is the yellow-backed duiker, the largest species, which is active during both day and night. [14]