Which Mammals Sleep The Least?

Sleep is a biological requirement for almost all animals, but the amount of sleep needed varies greatly across the animal kingdom. While larger animals tend to require less sleep than smaller ones, predators need more sleep than prey. Some of the mammals that sleep the least include the tallest animal on Earth, the giraffe, which sleeps for an average of 30 minutes to 4.5 hours per day, and the elephant, which sleeps for 2 to 4 hours per day.

Elephants sleep for around 2 hours a day

Elephants are known to be light sleepers, with wild African elephants sleeping for just about 2 hours a day, making them the mammals that sleep the least. In contrast, captive elephants in zoos or those in captivity sleep for 3 to 7 hours a day.

A study published in 2017 monitored two free-roaming African elephants and found that they slept for only 2 hours a day. They often sleep standing up, leaning against a tree or a termite mound to take some weight off their bodies. This is because, due to their large size, lying down for too long can cause pressure build-up on their internal organs.

The researchers used motion sensors fitted to the elephants' trunks to measure their sleep. The trunk is the most active part of an elephant's body, and it rarely stays idle while the elephant is awake. Therefore, when there was no movement detected for 5 minutes, it was assumed that the elephant was asleep.

The elephants slept in short bursts of polyphasic sleep, with each sleep session lasting between 1 to 2 hours. They also travelled long distances of around 30 kilometres during their sleepless periods, possibly to evade predators or poachers. Despite going days without sleep, they did not seem to compensate with extra sleep afterward.

Elephants' sleep patterns differ significantly from those of other mammals. For example, male lions can sleep for nearly 20 hours a day, with female lions clocking at least 15 hours. Tigers have similar sleep patterns. Carnivorous animals, such as big cats, tend to sleep more than herbivores like elephants and giraffes, which spend most of their time grazing for food.

Giraffes sleep for around 4.5 hours a day

Giraffes are the tallest mammals on Earth, and they sleep for the shortest amount of time. They sleep for around 4.5 hours a day, which is a little less than the grey whale (9 hours a day) and a little more than the elephant (2 hours a day).

Giraffes have a very unusual sleep pattern. They take a series of short power naps, just a few minutes at a time, throughout the day. They often sleep standing up—this is likely because it takes them a while to get up from lying down, and this makes them vulnerable to predators. They also tend to sleep in groups, taking turns to be vigilant while the others rest.

Giraffes sleep for less time than other mammals because they are large and need to spend a lot of time eating. They also need to be alert to protect themselves from predators.

Interestingly, sleep patterns in captivity, such as in zoos, are usually different from actual behaviour in the wild. This can skew the numbers reported for how much sleep each species needs per day.

Horses sleep for around 3 hours a day

There are a few reasons why horses don't need as much sleep as humans. Firstly, horses are prey animals, so they have evolved to be able to avoid and outrun predators. Being able to move quickly is a matter of life and death for horses, so they tend to prioritise this over sleep.

Secondly, horses are large animals, and larger animals tend to need less sleep than smaller ones. This is because they have slower metabolisms, so they don't need to sleep as much to restore their bodies.

However, just because horses don't sleep for long, doesn't mean they don't sleep well. Horses have evolved to be able to sleep standing up, thanks to their "stay apparatus". This is an anatomical mechanism that allows them to stay vertical even when they're not fully conscious. It works by locking the horse's kneecap with ligaments and tendons. This means that horses can snooze while standing, which is the safest way for them to catch some sleep. If they were to lie down, they would be more vulnerable to attacks.

That being said, horses do need to lie down to get REM sleep. This is the deepest, most restorative stage of sleep, and it's essential for keeping horses well-rested. Horses typically get this type of sleep in short bursts. They might lie down for 20 minutes, get up for a while, then lie down again later.

Horses also tend to sleep in groups, taking turns to sleep while others stand watch. This is another way they protect themselves from predators.

Walruses can go without sleep for 84 hours

Walruses are some of the world's most unusual snoozers. They can go without sleep for 84 hours, and they sleep anywhere—on land, at the bottom of the ocean, or floating. They can even hang from ice sheets using their tusks while they sleep.

Walruses are the only living species in the Odobenidae family. They are migratory animals, moving south to breeding areas every winter. Walruses feed on the ocean floor, mostly on clams. They are some of the largest pinnipeds and can weigh between 130 and 220 kg.

Walruses can sleep in water, even while resting and using their tusks to hang from ice floes. They can hold their breath for about four to five minutes while sleeping underwater, and they experience REM sleep during this time. On land, walruses settle into a deep sleep that can last for up to 19 hours. Their breathing pattern is regular during quiet sleep, with most pauses under 30 seconds, but becomes arrhythmic during REM sleep, with apneas lasting up to 160 seconds.

Walruses can engage in periods of almost continuous swimming for up to 84 hours. While other animals, including humans, can stay awake and active for this amount of time, doing so regularly and without forceful intention is unprecedented. This finding has led sleep researchers to reevaluate their views on the regulation and function of sleep.

Walruses experience "unihemispheric sleep," where one half of the brain sleeps while the other stays active. This may explain how they can withstand extended periods of activity. Walruses might also break the record for the greatest difference in total sleep time between individuals, as some appear to sleep much more than others.

In summary, walruses are remarkable mammals with unusual sleeping habits. Their ability to go without sleep for extended periods and their adaptability to sleep in various environments make them unique among their fellow creatures.

Dolphins sleep with half their brain asleep

Dolphins are fascinating creatures, and their sleep patterns are no exception. They have evolved to sleep with only half of their brain at a time, allowing them to stay constantly alert and vigilant. This phenomenon is known as unihemispheric sleep or unihemispheric slow-wave sleep (USWS). During this state, one half of the dolphin's brain enters a deep sleep, while the other half remains awake and alert, ensuring the dolphin can still breathe, monitor its environment, and watch out for potential threats.

Unihemispheric sleep is an incredible adaptation that enables dolphins to function with near-perfect accuracy, even after several days without full brain rest. This ability is crucial for dolphins' survival in the ocean, as they need to regularly surface to breathe and cannot afford to be completely unconscious for extended periods. By keeping one half of their brain awake, they can ensure they don't drown while still getting the necessary rest.

The sleeping dolphin's attentive side is used for more than just monitoring threats. It also helps the dolphin stay aware of its pod, and it signals when to rise to the surface for air. After a couple of hours, dolphins will switch, resting the active side of the brain and waking up the rested half. This pattern is often referred to as "cat-napping."

Dolphins aren't the only creatures that exhibit this behaviour. Many birds also sleep with one eye open and half of their brain awake, especially during long migratory flights. This allows them to maintain environmental awareness and control their wings while getting the necessary sleep. In both dolphins and birds, USWS enables them to rest in areas of high predation while still remaining vigilant and responsive to their surroundings.

The study of USWS is an important research topic as it challenges our understanding of sleep. Researchers are intrigued by the idea that sleep may not be essential for certain species, as otherwise, evolution might have eliminated this behaviour. Additionally, the ability to control sleep and wakefulness simultaneously in different parts of the brain is a fascinating aspect of neurobiology that warrants further exploration.

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