How animals sleep — and just how much rest they get — is as varied as the creatures themselves, but here are the top 10 drowsy critters (ranked by hours spent sleeping per 24 hours):

1. Koala: 20-22;

2. Sloth: 20;

3. Brown bat: 19.9;

4. Giant armadillo: 18.1;

5. Python: 18;

5. North American opossum: 18;

7. Owl monkey: 17;

8. Human infant: 16;

9-10. Tiger: 15.8;

9-10. Tree shrew: 15.8.

Since the amount of sleep required varies greatly from species to species, let's look at the other end of the spectrum.

The average giraffe, for example, sleeps for just 4.6 hours per day. For the most part, giraffes tend to sleep during the night, although they do get in some quick naps throughout the day.

They can doze off standing up as well as lying down, and their sleep cycles are quite short, lasting around 35 minutes and sometimes even shorter.

Elephants are another animal that sleeps very little. Some researchers have documented their total sleep time as just 2 hours per day.

(Interestingly, scientists can tell elephants are sleeping when their trunks stop moving.)

Elephants, like giraffes, likely only sleep for a few hours each day due to their massive body size and need to graze often. Predation risk may also play a role in how little they sleep, given how far they’ll travel while awake.

Some elephants have been observed traveling for nearly two days without sleeping at all.

During sleep, the human body cycles through four stages. Physical changes take place during each stage, such as decreased temperature and heart rate.

Different types of brain activity also occur during each stage, with more activity taking place during the fourth stage, called rapid-eye movement (REM) sleep.

In addition to the fluttering eyes behind eyelids, this sleep phase is also characterized by muscle twitching and waking-like electrical brain patterns. Although humans can dream during any phase of sleep, they are most likely to during REM sleep.

Many terrestrial mammals, including primates, and some reptiles, birds, and aquatic invertebrates also experience REM sleep. But again, the exact amount varies widely depending on the species.

Because elephants sleep so little, they don't experience REM sleep every day. In contrast, house cats can spend up to 8 hours a day in REM sleep.

Some animals, such as dolphins and whales, show little-to-non behaviors associated with REM sleep, although whales sometimes do exhibit some muscle jerking that might be representative of REM sleep.

Even though mammals and birds both spend considerable periods in REM sleep, REM seems to have become further enhanced in mammals. In birds, periods of REM last only a few seconds at a time, implying that the animal would be rapidly going into and out of dreams again and again.

Whatever the perceptual content of an animal's dreams, we can expect that they are just as chaotic (and, at times, repetitive) as our own.

If this is the case, we can only wonder how they interpret these strange experiences upon waking. Maybe they are just as confused and fascinated as we are.