Newborn Naps: Animals That Sleep Half Their First Day

Sleep is a biological requirement for almost all animals. However, the amount of sleep an animal needs depends on several factors, including age, body size, environment, diet, and the safety of its sleep site. Some animals have even developed the capacity to sleep with one half of their brain while remaining vigilant with the other—a behaviour known as unihemispheric slow-wave sleep (USWS). This allows them to stay alert for predators and other threats while still enjoying the restorative benefits of sleep.

Marine mammals, bird species, and possibly reptiles enter a half-on/half-off state, sometimes keeping one eye open during these intervals. Dolphins, whales, and fur seals in water do not have REM sleep like humans. Instead, they sleep unihemispherically, with one side of the brain asleep while the other stays awake. This allows them to swim, breathe consciously, and avoid predators and social contact during their period of rest.

Dolphins sleep with one eye open

Dolphins are fascinating creatures that have evolved to be unihemispheric , meaning they can function with one half of their brain while the other half rests. This is also known as unihemispheric slow-wave sleep (USWS). They sleep with one eye open, quite literally, as one eye closes and the other remains open.

Unihemispheric sleep is an essential evolutionary adaptation for dolphins. Unlike humans, whose breathing is automatic and regulated by the brainstem, dolphins breathe voluntarily. They must consciously decide to take each breath, and if they were to fall into a deep sleep like humans, they would lose this conscious control and drown.

Dolphins are able to rest one half of their brain while the other half remains awake and alert, allowing them to continue breathing, maintain awareness of their surroundings, and surface for air when needed. The open eye is connected to the active hemisphere, allowing them to remain vigilant and aware of potential environmental threats or changes. This is crucial for their survival in the wild.

Dolphins often rest motionless near the surface of the water or swim slowly, still able to breathe when needed. They periodically come up for air and keep an eye out for potential predators. They alternate which half of the brain is sleeping so that they can get the rest they need without ever losing consciousness.

Dolphin mothers with calves might sleep alongside their young, providing warmth and security. This social bonding is important for protection and social interaction.

The ability to sleep with one eye open is a testament to the adaptability and evolutionary prowess of dolphins, ensuring their safety and survival in their underwater homes.

Koalas sleep up to 22 hours a day

Koalas are native to Australia and are the only surviving member of their family, Phascolarctidae. They are tree-dwelling marsupials, and their closest living relatives are wombats. Koalas are born with underdeveloped bodies and spend their first six months in their mother's pouch.

Koalas are folivore specialists, and their diet consists mainly of eucalyptus leaves, which are poisonous to most animals. They can eat up to a kilogram of these leaves a day. Eucalyptus leaves contain toxic compounds similar to cyanide, and only koalas, possums, and gliders can eat them. To digest these toxic leaves, koalas have a specialised digestive system. First, they grind the leaves into a paste using their heavily ridged molars, then their liver isolates the toxins, and finally, specialised bacteria in their caecum break down the remaining residue.

However, this diet provides very few calories, and koalas obtain most of their water from the leaves, so they don't need to descend from their trees to drink. This means that they spend most of their time expending as little energy as possible and can sleep for up to 20-22 hours a day. They are mostly nocturnal and active at night, around dawn, and at dusk.

Koalas are solitary animals and live within a network of overlapping home ranges, which allows them to come into contact with other koalas for mating.

Elephants sleep for only 2 hours a day

Elephants are the lightest sleepers of any mammal, clocking in at just 2 hours of sleep a day. This is a stark contrast to their captive counterparts, who sleep for 3 to 7 hours a day. This is because wild elephants face more dangers and have a greater need to find food, leading them to prioritise staying awake over sleeping.

In 2017, Paul Manger of the University of the Witwatersrand in Johannesburg, South Africa, and his team monitored two female African elephant matriarchs in the wild for 35 continuous days to understand their sleep patterns better. They used motion sensors on the elephants' trunks, which are rarely idle while the elephant is awake, to determine when they were asleep. The elephants averaged just 2 hours of sleep per night, and this sleep was broken up into four to five short bursts, a pattern known as polyphasic sleep.

Interestingly, the elephants only slept lying down every few days, and there is no clear evidence that they need REM sleep. This calls into question the belief that REM sleep is vital for memory consolidation, especially since elephants are known for their good memory.

The amount of sleep an elephant gets seems to be related to its size. Bigger animals tend to sleep less, probably because they need to spend more time eating. Elephants can eat up to 300 kilograms of food a day, and their large size means that it takes a long time to get all that food into their mouths, leaving less time for sleep.

Despite their small sleep quota, elephants do not seem to be negatively affected, maintaining their impressive memory and awareness of their surroundings. This makes them an intriguing subject for further study on the impact of sleep on brain function and memory.

Killer whales can go a month without sleep

It is a well-known fact that all animals need sleep to survive. However, the amount of sleep required varies from species to species. Some animals, like the koala, sleep for most of the day, while others, like the African elephant, sleep for only about two hours a day. But one of the most fascinating sleep habits in the animal kingdom is that of the killer whale, which can go a month or more without sleeping.

Killer whales, also known as orcas, are large members of the dolphin family. They are marine mammals that live their entire lives in the water and never spend any time on land. So, how do they sleep without drowning?

The answer lies in the fact that killer whales are voluntary breathers. Unlike humans, who breathe without thinking about it, killer whales must consciously remember to breathe, even in their sleep. This means that their brain has to remain partially active while they snooze.

Killer whales achieve this by shutting down only one hemisphere of their brain at a time. While half of their brain is asleep, the other half remains conscious and alert, controlling vital functions like breathing and keeping watch for predators. This type of sleep is called unihemispheric slow-wave sleep, and it is a trait shared by many species, including other whales, dolphins, and even chickens.

During unihemispheric sleep, one eye of the killer whale remains open (on the "awake" side of the brain) while the other stays closed. This allows them to monitor their surroundings and surface for air when needed. After a period of about two hours, they switch the sleeping side to the other half of the brain so that both halves get adequate rest.

Interestingly, mother killer whales do not sleep or rest at all for the first month of their calf's life. They swim constantly, keeping a close eye on their offspring, who are vulnerable to predators and more prone to illness due to their lack of a thick layer of blubber. The mother and calf swim in an "echelon formation," with the calf eating, resting, and sleeping in the mother's slipstream, being towed along without having to exert energy to swim.

The ability of killer whales to go without sleep for extended periods is a remarkable adaptation to their aquatic environment. It ensures their survival by allowing them to stay alert for predators and maintain their body temperature while they build up their insulating blubber layer.

Sloths sleep for 10 hours a day

Sloths have a reputation for being lazy and sleeping all day long. However, this is just a myth. In reality, sloths sleep for about 10 hours a day, which is only a fraction more than humans.

The belief that sloths sleep for long periods likely stems from the fact that they are slow-moving, shy, and secretive creatures that are almost impossible to observe in the wild. They blend seamlessly into the rainforest canopy, making them difficult to spot. Additionally, they have favourite sleeping spots that they return to throughout the day and night, interspersed with short bursts of activity. This pattern of behaviour may have led scientists to believe that sloths can sleep for days on end.

The name "sloth" also means a form of "lazy" in many languages, and they have been described unfavourably in scientific literature as "the lowest form of existence". Furthermore, the myth that sloths are "stoned" due to psychoactive properties in the leaves they eat has contributed to their reputation for laziness.

The development of modern animal-tracking technology has allowed researchers to study sloths in their natural habitat and dispel the myth that they sleep excessively. Through projects such as the Sloth Backpack Project, scientists are using micro-datalogger technology to record the movements of wild sloths and determine their activity levels. By comparing this data with historical data collected through more invasive methods, such as electroencephalograms (EEGs) and electromyograms (EMGs), researchers estimate that wild sloths sleep for 8 to 10 hours per day.

It is important to note that sloths in captivity sleep significantly more than their wild counterparts, possibly due to stress. This may have contributed to the exaggerated estimates of their sleep duration.

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