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Non-human animals’ sleep patterns vary.

While scientists can’t say for certain that every animal sleeps, most creatures in the animal kingdom do, indeed, catch some zzz’s. But not all animals experience sleep in the same way.

For example, one animal may go through different stages of sleep than another. Insects and fish, unlike some birds and all mammals, don’t experience Rapid Eye Movement (REM) sleep. REM sleep is when most dreams occur.

The number of hours than an animal sleeps can also vary widely. Cats sleep, on average, for 15 hours a day, while rats clock up to 20 hours a day. Smaller animals, which often have higher rates of brain metabolism, tend to require more sleep, while larger animals generally get less sleep.

Even the way that an animal snoozes can be different. Interestingly, horses, cows, and giraffes can sleep while standing, but need to lie down for short stints for REM sleep.

Dolphins, as well as some other aquatic mammals and some birds, utilize something called unihemispheric slow-wave sleep. This allows one half of the brain to sleep while the other remains awake so that long, non-stop migrations are possible.

But some sleep behaviors are similar across the animal kingdom. For instance, a lot of animals sleep at night, when it is colder, and thereby avoid the extra energy expenditure that would be required if they were active in the p.m. Smart thinking!