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Written by: Nancy Moncrief
Wednesday March 4, 2015

KoKo, a standard poodle, settles down for a long nap.  Photo by K. Davis.

Everyone knows what it’s like to not get enough sleep.  Whether we stay up too late because we want to watch a movie, or we have to work on a report that’s due the next day, or maybe we can’t sleep because the neighbor’s dog is barking—we all know what it’s like.

But what’s enough?  Why do most adult humans need to sleep about eight hours each night? And why do our dogs and cats sleep even more?  For that matter, why do animals sleep at all?

By studying the sleeping habits of many different species of mammals, scientists have discovered a consistent pattern in sleep duration.  Mammals that eat other animals (carnivores) tend to sleep a lot (up to 20 hours a day).  Those that eat plants (herbivores) sleep the least-- some as little as 3 to 4 hours per day.    The sleep requirements of omnivores, which eat both plants and animals, are in the middle.  People (if they eat their vegetables the way they’re supposed to!) are omnivores, and most of us sleep about 8 hours each day as adults.

Horses and other grazing herbivores spend large amounts of time eating because they have to digest huge quantities of relatively low-energy plant food in order to get the energy they need to survive, grow, and reproduce.   In contrast, carnivores, such as tigers, eat a high-protein, high-energy diet, and they are able to get a lot of energy in a short time.

So, the amount of time that a mammal sleeps is related to its diet.  And this leads to the conclusion that one function of sleep is energy conservation.  That is, in order to conserve energy, animals sleep as much as they can when they are not looking for food.  If they don’t have to spend much time eating, they spend a lot of time sleeping.

Another function of sleep relates specifically to our brains.  There is evidence that some stages of sleep allow mammals to repair damaged brain cells.  Other stages of sleep also allow our brain’s communication systems to “rest” and “recharge” themselves.  So, sleep serves as a recovery period for our brains.

There are still many questions about why we sleep and what happens to our bodies and brains while we sleep.  More studies that compare and contrast information from a variety of animal species are needed to answer these mysterious questions.

Tags: Biology, Dr. Nancy Moncrief, Mammal of the Month, Mammals, Research and Collections