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A dog snoring all afternoon on the living room floor. Walrus snoozing on your stomach on a beach. Lions lying on the Serengeti. A hippopotamus dozing on a mud bank.
These sleepy scenes may cause people to wonder why these other mammals seem to sleep so much more than humans. Do they really need more sleep? Are they sleeping just because they can? Should humans get more sleep too?
Get ready for a surprising and fascinating answer to these questions:
Nobody knows.
Related: Can an animal survive without sleep?
Though constantly studied, sleep is one of the great mysteries that modern science has not completely solved. “I do not really know what it’s sleep,” he told WordsSideKick.com Dr. David Raizen, associate professor of neurology at the Perelman School of Medicine at the University of Pennsylvania.
Raizen said scientists have identified the relationships between sleep and animal function: certain types of sleep can increase a creature’s ability to fight disease or consolidate memories. However, these associations do not necessarily describe the ultimate purpose of sleep and can be misleading.
“A brown bat that sleeps 20 hours a day you would think would be a genius,” said Raizen, speaking of the idea that sleep is meant to serve learning and memory. Similarly, a 2017 study published in the journal PLOS One found that elephants sleep on average only two hours a nightbut elephants are known to be intelligent animals with great memories.
“This difference in the amount of sleep has been used to argue against a fundamental function of sleep,” Raizen said. How could sleep be so important if an animal like a elephant is it fully functional with only two hours while a typical human needs quadruple?
But Raizen thinks that sleep plays a fundamental, though still poorly understood, function. Sleep has been observed in every animal ever studied by science, Raizen said, making it as universal among life forms as energy supply. Studies have also shown that the bodies of various animals begin to break down if they are continually deprived of sleep, suggesting that sleep is essential.
So if it is essential, why isn’t it getting better and the quantity always similar, particularly among closely related animals, such as mammals?
One idea is that sleep in mammals has to do with body size and diet, according to a 2005 study in the journal Nature. In many mammalian sleep studies, scientists have observed that less sleep correlates with larger body size, and this correlation is stronger and more extreme among herbivores than it is among carnivores.
One reason 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 that carnivores eat, so much more needs to be swallowed. This may partly explain why an elephant may have evolved to survive only two hours a day, Raizen said.
However, the question is far from resolved. Sleep is complex, difficult to measure in wild animals and even more difficult to interpret.
In this sense, it is important to remember that some differences in observed sleep may not be what they seem. Just because a house cat sleeps 18 hours a day doesn’t mean it needs all that sleep to function, Raizen said. Some sleep is likely a matter of comfort, done when it is not safe for an animal to be around, when food availability is low, or simply because there is nothing else to do.
What about human sleep? Raizen said the most important thing is for people to listen to their bodies and sleep as much as they feel they need – which is about eight hours for most people, but it could be as little as five to as high as. 11.
Originally published in Live Science.
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