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:

• 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 six 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 on ice sheets with their teeth while they sleep.
• Orca calves spend the first months of their lives swimming around, and their mothers adjust their sleeping patterns to their calves' needs. Mothers stay awake during the first month or so while calves learn how to control their swim. As calves grow, the mother and clave pair will be observed at rest swimming together slowly, which allows them to rest.
• 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.
• Giraffes sleep an average of 35 minutes during the wet season. Giraffes in the wild will only sleep after the sun sets. Because getting up from the ground takes them a little time, lying down makes them more vulnerable to predators such as lions. For this reason, giraffes will spend the nights in groups. Once in the resting area, Giraffes  will take turns switching between being active and vigilant or laying and taking the sleep posture.  There is always a couple of active sentinels on watch while others rest so you could say they got each other’s necks in the best possible way. 
• 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 daily eating 100–400 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 daily, most of which occurs overnight. Cows enjoy about one hour of REM sleep. Because they need to spend most of their days ruminating, and when they do sleep, they sleep in short bouts.  A cow's sleep cycle consists of 1 non-rapid eye movement (NREM) bout about 5-8 minutes long, followed by 1 REM bout of about 3-4 minutes before the cow wakes up.
• Ants do rest and become less still, lose some sensitivity, and some might even hibernate. However, ants do not sleep or dream the same way we do, simply because they do not have a complex brain or nervous system able to experience the waves.  
• 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; sleeping is a tactic to conserve energy as their eucalyptus-based diet is not very calorie dense. 
• Pocket mice weigh as much as 5 paper clips  (7-10 g). 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, burying it, and looking for another tree. Sloths don't always move slowly. They can move as fast as a cat when in danger. They move slowly to conserve energy.
• Brown bats sleep upside down. They spend almost 20 hours/day asleep.
• Armadillos spend 16-18 hours a day sleeping in burrows.  Armadillos have even been observed falling asleep while eating! 
• Opossums sleep on average 19 hours/day and their individual sleep episodes can last between 1-4 hours. 

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 belly up 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 belly up. 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 maybe it's trying to cool off. If your dog cuddles with you, it shows 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. If you have noticed a change in your dog’s sleeping habits it is best to get them checked by a vet ensure there is no medical reason behind this change. 

Cats are known to sleep a lot, in fact, cats spend an average of 12-13 hours and they tend to sleep more when it's rainy or cold up to 16 hours a day! A cat’s sleep pattern is described as polyphasic, meaning that rather than one single long sleep like ours, they sleep multiple times a day. If you think about it, you often find your kitty sleeping! Most of this time, cats are in a slow wave or light sleep that allows them to rest and conserve energy. Cats in a light sleep can spring into action at any moment. Cats, however, also experience REM (rapid eye movement, the stage of sleep where dreaming occurs), just like humans, and they also dream. You can tell when a cat is in REM when its body and face twitch.

You might have noticed that your cat seems to be more active while you sleep. This is because cats are crepuscular creatures, as their ancestors hunted at twilight or low light conditions. Their eyes are equipped with a layer of specialized cells that allows them to reflect the light you have probably already noticed how they shine at night.  Besides this fantastic vision, cats also have incredible sense of smell and fantastic hearing. Some Cats may snore, and certain breeds do so more often because of the structure of their faces, such as Persians and Himalayans.

Recent discoveries have concluded that while fish do not experience Rapid Eye movement or REM, and slow wave sleep, they do experience 2 distinct stages when they rest. The scientists of the Center for Sleep Sciences and Medicine, Department of Psychiatry and Behavioral Science of Stanford University, term them “slow bursting sleep” and “propagating wave sleep”.   When they rest, many fish will reduce their activity levels and metabolism, which saves energy and helps restore the body. They enter a state where they're very relaxed and only occasionally move a fin to keep their balance. Fish and sharks will still 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 with eyelids but 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 during the day and rest at night, while others do the opposite.

One fish species, 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, it 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, creating a large mucus 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.

While it was previously suggested that reptiles don't experience REM sleep, reptiles do experience REM sleep, the reality is that we have only concluded that they do experience two distinct stages, a NREM sleep-like stage, and a REM sleep-like stage. Each state of this cycle lasts only about 80 seconds in lizards. And 80-90 seconds in the Australian bearded dragon, however, in the Argentine Tegu the duration of stage one is about 6 times longer than the duration of stage 2. The cycle duration seems to vary between species and is influenced by environmental temperature.

And while no one can confidently conclude that they do have 2 diverse cycles, the physiological differences between the reptile and mammalian brains and the fact that reptiles can not control their body temperature makes it impossible for us to know with our current technology if these are analogous to the mammalian REM sleep cycles. But if we are comparing only the cycles’ frequency, human cycles last from 70 to 120 minutes, while a cat's is 15- 30 minutes. Lizards will go through 350 full cycles every night instead of 4 or 6 for 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 predator attack. 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 can still 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 diurnal 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 30 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 they're in REM sleep. In humans, REM sleep is a sign of dreaming, so octopuses may dream too.

 Insects don't sleep as humans do and they definitively do not dream. But we know for a fact they rest. 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 requires rest.  In general, insects will relax, stop moving, and some will exhibit external signs of resting  such as cockroaches that fold their antennae when they rest his state is called torpor, which isn't exactly sleeping, but similar.

Male Fruit flies will rest for up to 10 hours, while females only 5 hours per night,this might be attributed to the higher energy need of females for egg production.  . While resting, they don't move, except for occasional twitching, and they don't respond to sensory stimuli. Interestingly some fruit flies have been observed to rest for less than 20 minutes a day.

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, flies 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), making them fall over.

Some birds can also sleep standing up, for example, flamingos have to sleep standing up as they live in caustic salt flats.

Some animals will hibernate for weeks or months when food becomes scarce. In cold weather, it's called hibernation in mammals and brumation in reptiles.  But it can occur in warm weather as well (called estivation). During brumation and hibernation, the heart rate and breathing rate both drop significantly, and 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 usually eat a great deal beforehand to develop fat stores to live off while hibernating.

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

Animals that hibernate or brumate include:

• prairie dogs
• skunks
• bears
• bumblebees (only the queen)
• snakes
• box turtles
• land snails
• bats
• wood frogs
• fat-tailed dwarf lemurs
• Chipmunks (topor)
• 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 seems to help 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 in groups called sleeping huddles. Each huddle shares a branch on the tops a tree at night, although baboons spend most of the day at land. Most animals will sleep in several sessions throughout the day (polyphasic), whereas humans will usually sleep in one session (monophasic).

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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