Larger animals tend to need less sleep than smaller ones, and those that graze sleep less than carnivores that eat big meals. Here are the animals that need the least amount of sleep:

• Bullfrogs go without sleep for months at a time! Though they close their eyes and rest, they remain alert and will respond to painful stimuli and show changes in respiration. They do sleep deeply during hibernation in the winter.
• Impalas almost never sleep. Male impalas in particular rarely let their guard down as they're watching the herd.
• Alpine swifts fly from Switzerland to West Africa for 6 months straight without stopping! It's unclear if they sleep or rest while in flight.
• Walruses can go for 84 hours with no sleep! When they do sleep, they can sleep anywhere on land, on the bottom of the ocean, even floating. Sometimes they'll bite down onto ice sheets with their teeth while they sleep.
• Orca calves spend the first few months of their lives swimming around, even if their parents are asleep. It's believed that this keeps them safe from predators and keeps them warm as they haven't developed blubber stores yet.
• Dolphins sleep with half of their brain asleep (called unihemispheric slow-wave sleep or USWS), as many birds do, which means that only one of their eyes will work (the one opposite the hemisphere that is awake). The one eye keeps an eye out for predators, and they can come to the surface to breathe. Some dolphins will swim while they sleep, others will float and look like a log. Dolphins will sleep with one side of their brain asleep for 2 hours, then switch and sleep with the other side of their brain.
• Giraffes sleep 30 minutes to 2 hours per day, often broken up into smaller sections. Because getting up from the ground takes them a little time, lying down makes them more vulnerable to predators such as crocodiles and lions. For this reason, they will often sleep standing up and may rest their head on their rumps. Until the 1950s, researchers believed that they didn't sleep at all.
• Horses sleep 2.5 hours/night and tend to nap for 15 minutes at a time.
• Deer need 3-4 hours in 24 hours.
• Elephants sleep 3-4 hours per night. Because of their size, they need to spend up to 18 hours per day eating 200-600 pounds of food/day! They sleep standing, leaning on a tree or termite mound, or lying on their side. If they lie on their side their sleep is less than 30 minutes, preventing their internal organs from being crushed.
• Sheep sleep less than 4 hours per night, and sleep huddled together as some other flock animals do.
• Goats and cows sleep 4 hours per day, divided into short periods. Most of their days are spent looking for food, and when they do sleep, they sleep in groups and take turns to keep watch for predators.
• Ants take one-minute naps several times equaling 4 to 5 hours per day. However, the queen ant sleeps up to 9 hours/day.

• Koalas sleep up to 22 hours/day; they don't sleep because they need to - they do it for pleasure!
• Pocket mice weigh as much as 5 paper clips. These little rodents sleep up to 20 hours/day.
• Sloths are slow-moving creatures that sleep up to 20 hours/day in a tree, only coming down to relieve themselves, then burying it, then looking for another tree. Sloths don't always move slowly. They can move as fast as a cat when in danger. They move slowly just to conserve energy.
• Brown bats sleep upside down. They spend almost 20 hours/day asleep.
• Armadillos sleep a lot due to the weight of their shell, which is three times its body weight! They sleep 19 hours per day.
• Opossums sleep on average 19 hours/day, possibly due to all of the climbing and jumping they do in trees.
• Lemurs sleep about 16 hours/day. Their need for lots of sleep may be due to the high temperatures of the tropics that they inhabit.

A human infant sleeps 16 hours/day (66.6% of the day), an adult 8 hours (33.3%), and an elderly person 5.5 hours/day (22.9%).  How does that compare to animals? According to the University of Washington:

Dogs go through the same sleep stages as humans; they just spend less time at each sleep stage. And no one knows why, but big dogs dream longer than little dogs, and little dogs dream quickly and frequently.

The position a dog sleeps in can tell you about how they're feeling. A dog sleeping on its side feels safe as its vital organs are exposed. If they're sleeping somewhere new or around people who they're not familiar with, they may not sleep on their side.

If a dog sleeps curled up in a ball, it may be because they're not in a comfortable environment and feel more secure protecting their organs, and they can get up faster, or this may conserve heat.

Puppies will often sleep on their stomach so that they can get up quickly and not miss any playtime! If a dog sleeps on its back, it is either very comfortable, as its organs are the most exposed, or it's trying to cool off. If your dog cuddles with you, it's showing you affection and trust.

Many dogs will circle before lying down, which is what their ancestors, wolves, would do to trample grass down to make a comfortable bed. Digging is something their ancestors would do to keep them warm in the winter and cool in the summer.

If your dog sleeps during the day, it may be due to boredom. If there's any kind of noise and their ears perk up, it means that they are just waiting for something exciting to happen and aren't in deep sleep.

Cats experience REM (rapid eye movement, the stage of sleep where dreaming occurs) and non-REM sleep, just like humans, and they also dream. You can tell when a cat is in REM when its body and face twitches.

An interesting fact about cats is that even when they're sleeping, they can wake up instantly, and they're ready to run or fight.

Cats are normally nocturnal as their ancestors hunted at night, and they also have very good hearing and sight.

They're known to sleep a lot, and it's thought that this is so they can conserve energy to be able to spring into action at any moment. And they tend to sleep more when it's rainy or cold.

Cats may snore, and certain breeds do so more often because of the structure of their faces, such as Persians and Himalayans.

Fish don't go to sleep as mammals do, and they don't go through REM sleep, but almost all fish rest. Many fish will reduce their activity levels and metabolism, which saves energy and helps restore the body, and enter a state where they're very relaxed and only occasionally move a fin to keep their balance. Sharks must remain swimming while resting because they need to continually pass water over their gills for a constant source of oxygen.

Fish don't close their eyes while asleep because they don't have eyelids. Sharks are the only fish that have eyelids, but they only close them while attacking prey.

Some fish will find a safe place to rest, such as an underwater cave, between rocks, or burrowed into the sand, and some will float in place. While asleep, fish will remain somewhat alert to avoid predators. Many fish are active in the day and rest at night, while other fish do the opposite.

The only fish that don't sleep are blind, cave-dwelling fish and deep water fish who swim continuously. The theory is that sleep is a way to process information and visual stimuli from the day. Since these fish aren't processing any information, there's no need for rest.

One species of fish, zebrafish, can also suffer from sleep deprivation like humans. If a fish can't sleep due to spawning, migration, threats from predators, or some disturbance in their environment, they will make up for the lost sleep as soon as things return to normal.

Parrotfish have a unique way to protect themselves during their rest periods. They go to the ocean floor and create a large snot bubble around themselves. The predator they're protecting themselves against is a blood-sucking parasite called a gnathiid. During the day they can seek the service of cleaner fish, such as the bluestreak cleaner fish that will eat the gnathids. But at night the cleaner fish aren't active.

Unlike fish, reptiles do experience REM. Whether they dream during REM is unknown, though there is some neural activity. Dr. Gilles Laurent told Scientific American, "If you forced me to speculate and to use a loose definition of dreaming, I'd speculate that those dreams are about recent notable events: insects, maybe a place where there are good insects, an aggressive male in the next terrarium, etc."

The sleep cycle of lizards only lasts 80 seconds, compared to a human's, which lasts from 80 to 110 minutes, while a cat's is 30 minutes. Lizards will go through 350 full cycles every night, as opposed to 4 or 5 for humans. But they seem to go through the same cycles as humans.

Like dolphins, many birds will sleep with one eye open, and one-half of their brain awake (unihemispheric slow-wave sleep or USWS). If they open their right eye, it's the left half of the brain that's awake. They can control just how much of their brain is asleep by how wide they open or close their eye! The amount of sleep and which part of the brain sleeps is determined by which part of the brain has been most active during the day. The parts that were the most active received the most amount of sleep.

Another factor in the amount of USWS is the risk of attack from predators. Ducks near the perimeter of the flock are more likely to first react to threats from predators than ducks in the middle of the flock.

USWS keeps a bird safe from predators while they sleep. Some birds can even sleep while flying, especially during long migrations, and they're still able to navigate. Ducks and geese that fly in J and V formations will use USWS when not in the lead position of the group.

Nocturnal birds will wake up when the sun sets and hunt during the night, but most birds follow the same diurnal pattern as humans and sleep at night.

For birds that sleep on a perch, when they place weight on their feet, the tendons in their feet tighten which closes their feet, giving them a good grip on the spot they're holding onto. Other birds will sleep inside of trees, in ground cover, or in the water.

Scientists aren't sure whether birds dream or not, but in male zebra finches the neurons in the robustus archistratalis (song system of their brains) show spontaneous bursting patterns during sleep, which suggests that they are replaying songs that they've played or heard during the day.

Octopuses are invertebrates, meaning that they don't have a spine. Octopuses are highly intelligent and go through sleep cycles, including REM sleep.

When they sleep, they go into their homes, narrow their eyes, and become still for long periods. Approximately every 15 minutes they will twitch their tentacles and rapidly change color. Scientists used to believe that they were waking up to check for predators but now believe that they're in REM sleep. In humans, REM sleep is a sign of dreaming, so it's possible that octopuses dream too.

The answer most likely is yes, though they probably don't sleep as humans do. They don't have eyelids, so it's not easy to tell if they sleep, but they do have a central nervous system and circadian rhythms, which seem to be important signs of a life form that require sleep. In general, insects will relax, stop moving, and some will exhibit external signs of sleep, such as cockroaches that fold their antennae when they sleep. This state is called torpor, which isn't exactly sleeping, but similar.

Fruit flies will find a place to rest for 2.5 hours. While resting, they don't move, except for occasional twitching, and they don't respond to sensory stimuli.

Fruit flies that can't rest will then rest longer than normal to make up for the lost rest. Similar to humans, they will be slower at learning when tired, such as taking longer to find their way through a maze. In one experiment fruit flies were denied rest for so long that they died. In research with honeybees, the bees were unable to perform normal tasks when rest-deprived.

Examples of insects that rest during the day are cutworms that eat leaves at night to avoid predators, crickets, and bed bugs that feed on humans who sleep at night. Many other insects rest at night.

Insects have circadian rhythms like other animals and humans, responding to changes in light and temperature. And just like humans who are given caffeine, insects' ability to rest is also affected.

Giraffes, elephants, camels, horses, and cows can sleep standing up by locking their legs so their muscles aren't engaged, which is called a stay apparatus. In this way they're less vulnerable to predators because they can more easily run away, not having to take a few seconds to stand up before running. They aren't able to engage in REM sleep in this posture however, so they also need to lie down. REM sleep causes muscle atonia (a sort of muscle paralysis) which would make them fall over.

Birds can also sleep standing up, which is usually when they can't find a comfortable place to sleep. Flamingos have to sleep standing up as they live in caustic salt flats.

Animals will hibernate for weeks or months at a time when food becomes scarce. In cold weather, it's called hibernation, but it can occur in warm weather as well (called estivation). During hibernation, the heart rate and breathing rate both drop significantly, the core temperature drops, yet the animal will have brief bouts of wakefulness. During this period the animal won't drink, eat, or relieve itself. Hibernating animals will usually eat a great deal beforehand to develop fat stores to live off while hibernating.

Surprisingly, for some animals hibernation isn't restful as they need to sleep after hibernation!

Animals that hibernate include:

• prairie dogs
• skunks
• bears
• bumblebees (only the queen)
• snakes
• box turtles
• land snails
• bats
• wood frogs
• fat-tailed dwarf lemurs
• chipmunks
• hedgehogs

In general, large mammals sleep less than small ones, which is believed to explain about 25% of the difference in sleep amounts for different mammals. Larger mammals also have longer sleep cycles. Other factors include basal metabolism and brain mass.

Animals with large brains require more REM sleep, as REM sleep helps the brain consolidate memories and learn.

Protection against predators is an important factor in how animals sleep, and for how long. For example, as mentioned, many birds will sleep with one eye active and half of their brain awake so they can fly to safety if attacked.

Carnivores tend to sleep more than herbivores, most likely because herbivores have to spend many hours eating every day whereas carnivores can take in a large number of calories in one meal.

Some species such as lions sleep in short periods during the day and at night so they're able to take advantage of feeding opportunities.

Otters will float on the water lying on their backs, often wrapping themselves in seaweed. Floating in water keeps them safe from predators on land, and wrapping themselves in seaweed keeps them from floating away. As many as 100 otters can be seen wrapped in seaweed, which is called an otter raft. Otters will often link paws together to keep from floating off.

Chimpanzees will build sturdy nests lined with leaves in trees to sleep in.

Meerkats will sleep in piled groups of up to 40 meerkats with the alpha males and alpha females as far away from possible threats. Communities of meerkats are called gangs or mobs and use closeness to keep each other warm.

Walruses can sleep on land or in the water. To keep their head above water when sleeping, they fill their pharyngeal pouches, located on each side of their esophagus.

Some speculate that great white sharks face the current while asleep so they don't have to keep moving, as the current will pass water over their gills providing them oxygen without needing to swim.

Snails can go into a hibernation period of up to 3 years! A famous story involves a British museum officer who attached an Egyptian desert snail to an identification card. Four years later there were traces of slime on the card and when the staff member removed the shell from the card, the animal crawled out! Snails secrete a layer of mucus around them before hibernation to prevent them from dying due to dehydration in a process called estivation.

Guinea baboons sleep on the tops of trees on their heels so they can run off immediately if attacked.

Most animals will sleep in several sessions throughout the day (polyphasic) whereas humans will usually sleep in one session (monophasic).

Cows like to sleep close to their relatives and their position is determined by their rank in the social hierarchy.

Male malachite sunbirds sleep with their bright yellow pectoral tufts fluffed up to give the impression to predators that they're looking into the eyes of a large mammal.

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