Why do animals sleep

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Think Fast! Tigers are Grrrrreat! Who Needs Sleep Anyway? Why Did the Turtle Cross the Road? Why Don't Woodpeckers Get Headaches? Written by: Lindsey O'Connell. Essential: required, or necessary. Evolution: is any process of growth, change or development over time. Bonobos , chimpanzees , gorillas , and orangutans all also build sleeping platforms in trees, away from predators and insects, a jungle version of a bed.

Gorillas sleep for 12 hours but orangutans get around the same eight hours that humans do. In some other primates, as in most mammals, sleep is polyphasic, with several alternating periods of sleep and activity in a hour cycle.

Dogs have wake-sleep cycles of about 83 minutes and get a little more than 10 and a half hours of sleep per hour cycle. The reason great apes have such long, luxurious sleep compared to the fitful, shorter sleeps of their monkey cousins has to do with those sleeping platforms. Monkeys have to balance on hard branches where they are easily awakened by potential danger or other monkeys—which is helpful to them but not good for extended sleep.

When apes started getting bigger, the branches they once slept on could no longer hold their weight—so they started building something that would. Being able to lay down, away from the dangers of predators and other distractions allowed them to sleep longer, more securely, and more deeply.

A study showed that orangutans do, indeed, sleep better than their baboon cousins. Dolphins, meanwhile, can stay alert with half of their brain while the other half can fall into a deep sleep.

This enables dolphins to sleep with one eye open, looking for predators. This sleeping pattern—which dolphins share with other cetaceans, manatees, eared seals and some birds—is called unihemispheric slow wave sleep, a deep state of sleep in which rapid eye movement or REM sleep does not occur.

REM sleep is the sleep state in which the brain is most active, breathing becomes more rapid, and most muscles become temporarily paralyzed.

The importance of REM sleep has been a subject of scientific debate concerning how much of a role it plays in memory and learning. Dolphins are highly intelligent but possibly never experience REM sleep, says David Raizen , a neurologist at the University of Pennsylvania, because if they did experience the same muscle paralysis as terrestrial animals they would sink to the bottom of the ocean and drown.

Frigate birds fly for months over the ocean and can engage in both regular sleep and use half their brain at a time to sleep during soaring or gliding flight. They sleep only while on rising air currents which allow them to gain altitude and keep them from falling in the water during the short second bursts of total sleep they grab while flying.

On land, they get about 12 hours a day in one-minute bursts. Fruit flies, though, need their shuteye, Raizen says. Sleep has been observed in every animal ever studied by science, Raizen said, making it as universal across lifeforms as energy intake. Studies have also shown that various animals' bodies begin to break down if they're continuously sleep deprived, suggesting sleep is essential.

So if it's essential, why isn't more always better and the amount always similar, particularly across closely-related animals, like mammals? One idea is that sleep in mammals has to do with body size and diet, according to a study in the journal Nature. Across many studies of mammalian sleep, scientists have observed that less sleep is correlated with larger body sizes, and this correlation is stronger and more extreme among herbivores than it is among carnivores.

A reason for this may be that the larger an animal is, the more calories it needs, and the more time it needs to spend eating. Herbivores tend to rely on food that is far less calorie-dense than the food carnivores eat so need to gulp down much more.

This could partly explain why an elephant may have evolved to survive on only two hours a day, Raizen said. However, the matter is far from settled. Nelson R. Protein and fat metabolism in hibernating bears. Federation Proceedings, 39 12 , — Dausmann, K. Physiology: Hibernation in a tropical primate. South, K. Animals, 10 8 , Melvin, R. Torpor induction in mammals: Recent discoveries fueling new ideas.

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