On the Internet, you can find particulars about animals that don’t sleep. However, as a rule, these supposedly ever-awake creatures still have sleep time – not as regular schedule human sleep.
In nature, most animals do not have a home where they can hide and sleep in pleasure. Sleep in animals is akin to altered consciousness, but they still have higher electrical activity in the brain.
Many animals have learned to sleep standing up – horses, giraffes, ostriches. The sleep of marine mammals is also short and intermittent, maybe hours per day, so they cannot drown.
In animals, the function of sleep is to maintain body temperature, regulate biological function or be safe from environmental conditions.
Today, we will showcase animals that don’t sleep or possess a sleep-like state or whose sleeping habits are sensitive and very short periods of time.
Table of Contents
1. Jellyfish- Ancient Sleepless Creatures
| Scientific Name | Scyphozoa |
| Phylum | Cnidaria |
| Sleep Duration | 10-20 seconds |
| Sleep During | Night |
| Unique Feature | Absence of Brain |
For many, these sea creatures don’t show signs of sleep due to a lack of central nervous system or brain activity. But research published in Current Biology in 2017 indicated that jellyfish also need sleep.
Analysts from the California Institute of Technology in Pasadena observed 23 Cassiopeia jellyfish continuously for six nights and six days.
The jellyfish in question, a stationary type Cassiopeia, which usually lives in mud and mangrove swamps, actually appeared to sleep during certain day phases.
The researchers manually counted individual impulses (heart rate) produced by contracted or relaxed phases at a constant rate and found that these impulses were less frequent during the night.
They found that at night the pulsation of the jellyfish dropped to 39 beats per minute and returned to 60 beats per minute during the day.
To ensure that this slowing down of the pulsation represented sleep, the scientists built a special elevator that lifted the jellyfish from the bottom, where they preferred to be, to the water’s surface.
Sleeping jellyfish in this situation, like only awakened people, reacted slowly. If the biologists raised jellyfish in an awake state, then they immediately sank to the bottom.
The sleep researchers realized that the food that fell into the tanks could awaken the jellyfish through the following experiments with the addition of food.
Biologists tried keeping jellyfish awake for 6 or 12 hours by spraying them with water. The next day, the jellyfish activity, as expected, was reduced and recovered after they were allowed to sleep. Finally, the sleep-wake rhythms of jellyfish were altered by melatonin, as in humans and mammals.
According to Paul Sternberg, lead author of the study and a researcher at the California Institute of Technology, this new research pushes the study of the origin of sleep much further down the evolutionary tree of life – before a proper centralized nervous system appeared.
2. Giraffe- Tallest Standing Sleeping Mammal
| Scientific Name | Giraffa |
| Phylum | Chordata |
| Sleep Duration | 4.6 hours a day |
| Sleep During | Day and Night |
| Unique Feature | Can sleep standing |
Of all mammals, giraffes have one of the most negligible animal sleep requirements. On average, giraffes sleep 4.6 hours a day. The mammalian sleep nature of giraffes has 10-50% REM sleep.
Since giraffes wake up very often at night, the duration of their deep sleep in total does not exceed 30 minutes. Therefore, they also take a nap during the day to relax fully.
At night, giraffes sleep lying down with their necks bent to rest their heads on the underside of their hind limbs. They go to sleep very quickly – in 15-20 seconds.
During the day, the giraffe is almost constantly in an upright position. Therefore, it also sleeps while standing with its eyes closed. The giraffe sometimes sticks its head between two branches that support it to avoid falling.
In 2021, a study led by an international team of researchers from the University of Copenhagen and Northwestern Polytechnic University in China found the genes responsible for this unique giraffe feature.
The FGERL1 gene led to giraffes needing less sleep than other animals because it’s not easy for them to get up from sleep, let alone get up and run away from a predator.
“We found that key genes that regulate circadian rhythm and sleep were highly selected in giraffes, possibly allowing the giraffe to interrupt the sleep-wake cycle more often than other mammals,” said one of the study’s lead authors from the Department of Biology at the University of Copenhagen Rasmus Heller.
He also added that giraffes are generally very alert and use their height advantage to scan the horizon using their superior vision.
3. Elephants- Biggest Animal that sleeps only 2 hours
| Scientific Name | Loxodonta |
| Phylum | Chordata |
| Sleep Duration | 2 hours of sleep per night |
| Sleep During | Day and Night |
| Unique Feature | Can go without sleep time for two days |
Elephants are the most “sleepless” species of Animal Kingdom. Based on a research study published in the journal PLOS One, these giant land mammals sleep just two hours a day, while their brains are not damaged or suffer from lack of sleep.
Paul Manger of the Witwatersrand University in Johannesburg, South Africa, one of the scientists involved, said, “In captivity, elephants sleep about 4-6 hours, but we tracked their sleep in natural conditions. It turned out that wild elephants sleep only two hours a day, which is a record low figure for mammals. It is probably due to their large size. On top of that, elephants seem only to dream once every 3-4 days.”
To test this hypothesis, they made an expedition to the territory of one of the reserves of Botswana, where African elephants live, found several larger animals, and attached motion sensors and a GPS receiver to their neck and trunk.
Collecting these sensors in a few weeks, zoologists have uncovered many new and exciting facts about the life of elephants.
First, it turned out that elephants can go most hours without sleep of all animals. All the individuals studied by scientists slept for about two hours a day, often going up to two days without sleep if they were recently disturbed by predators or poachers.
In addition, Munger and his colleagues proved that elephants could sleep both standing up and sinking to the ground, something scientists have long argued about, based on observations of elephants in zoos.
Elephants fell asleep in a strictly defined period – long before the sun rose in the wee hours. According to Munger, this is not because of daytime and night cycles but of humidity, air temperature, and other natural characteristics.
4. Horse- Fastest Animal with Little Sleep
| Scientific Name | Equus caballus |
| Phylum | Chordata |
| Sleep Duration | 2 to 5 hours a day |
| Sleep During | Day and Night |
| Unique Feature | Can sleep standing |
Horses can only sleep two to five hours a day. They can sleep in short bursts that last about 15 minutes. Similarly, they may also sleep standing up or lying down.
Young horses tend to sleep more than adults. Animals sleep best in groups for added protection, so a lone horse cannot sleep as its primary instinct is survival.
While sleeping standing up, these elegant animals do not react to anything, except that the tail and ears sometimes twitch. Therefore, it seems that this is their typical form of sleep.
Due to the unique structure of the knee joints (if necessary, they can be blocked, fixing the ligaments and bones), the animal can evenly distribute the body weight between the four legs, almost without feeling its heaviness during muscle relaxation.
The animals doze off in this state with a slightly arched lower back, lowered head, tail, and slightly drooping lower lip. But it’s hard to call it deep sleep because how a horse usually sleeps can vary.
They must always be alert in the wild so the predator cannot take the herd by surprise. Therefore, horses mainly restore strength due to drowsiness – they stand on outstretched legs and listen sensitively to the surrounding sounds in a half-sleep.
For several days they can get by with such a rest, but then they need to sleep generally for at least 2-3 hours. In the stables, horses feel safe, allowing themselves to sleep for real more often.
Experts say that horses have dreams – they move their eyeballs under their closed eyelids, snort excitedly, and move their legs as if they are running somewhere.
But some unfortunates are not threatened by normal sleep for several years in a row – these are the leaders of wild herds who can only doze lightly; otherwise, young rivals will cover their females.
5. Deer- Alert Even During Sleep
| Scientific Name | Cervidae |
| Phylum | Chordata |
| Sleep Duration | 12 hours a day |
| Sleep During | Day and Night |
| Unique Feature | Can sleep standing |
Deer are satisfied with just a few hours of sleep – they are not granted more as flight animals. To be precise, a deer doze off for about 30 seconds to 3 minutes repeating the cycle for about 30 minutes.
They don’t sleep all the time either, but only for a few minutes because they can’t afford to sleep for long periods because of the large number of enemies.
In a deep sleep, deer have their eyes closed, heads on the ground, or hind legs. A deer that wants to settle down to sleep first makes a bed with its forelegs.
Then it comes down on the front foot roots, sits on the right or left thigh, and folds the forelegs. There is always only one front or hind leg on one side under the body.
The deer chews the cud, dozes, or sleeps in this posture. When dozing, the head stays high; occasionally, the deer even continue to chew while dozing.
Even during these periods of sleep, they are cautious not to fall prey to predators. Deer’s primary concern is survival, making them sleep efficiently and quickly.
6. Whales- Mammals that Sleeps on Water Surface
| Scientific Name | Cetacea |
| Phylum | Chordata |
| Sleep Duration | 10-15 minutes |
| Sleep During | Day and Night |
| Unique Feature | Can go without sleep for a month, sleep with one eye open, and largest mammals |
These marine mammals can afford to sleep for long periods as they can easily drown due to their enormous body size. They sleep, swaying upright on the water’s surface or shallow water, for 10-15 minutes.
Scientists suspect that they require the least amount of sleep among mammals, accounting for less than 7% of their daily activity.
One of the fascinating behaviors includes whales do not need to sleep for 24-hour periods.
In fact, sleep requirements for whales for periods might not arrive until a month. They sleep almost on the surface of the water.
The body of a whale has a high content of soft adipose tissue, so the animal’s weight slightly exceeds the specific gravity of water. It also lacks a breathing reflex like we humans and other animals have.
We breathe automatically, even when we are asleep and even when we are unconscious. But whales must take every breath they take consciously – which wouldn’t work if they fell completely into a deep sleep.
So a sleeping whale slowly sinks to a depth and, after a while, strikes its tail in a dream, which again rises to the surface. Having inhaled the air, the animal slowly descends into the depths until the next blow with the tail.
These animals have also developed an exceptional strategy: They alternately only sleep one-half of their brain. The other stays awake (and always keeps one eye open), prompting the whale to catch its breath in time.
During the rest or sleep phases, the behavior differs from whale to whale species. Pilot whales, for example, swim very slowly at the surface of the sea, diving briefly and immediately coming up again to take a breath.
In the case of sperm whales, it seems that both hemispheres of the brain sleep while they stand vertically in the water just below the surface (usually with their heads up).
They consciously hold their breath and reappear at the right time – a built-in “breath alarm” that works amazingly well!
7. Dolphin- Mammal who Sleep with One Side of the Brain Active
| Scientific Name | Delphinus |
| Phylum | Chordata |
| Sleep Duration | 8 hours |
| Sleep During | Night |
| Unique Feature | Sleep with one eye open and sleep with one-half of their brain active |
Typically, dolphins only go to sleep on one side of their brain, as they use the other side of their brain for swimming and for watching for predators.
Dolphins swim slowly or lie still on the surface during these sleep phases. In the case of captive examples, dolphins were found sleeping at the bottom of the pools.
In 24 hours, dolphins sleep about 8 hours, 4 for each brain hemisphere. There are three reasons why dolphins and other aquatic mammals have adopted this way of sleeping.
Firstly, a high-alert sleep strategy allows them to control their breathing, preventing them from sinking and surfacing when needed. In addition, dolphins remain alert to the different dangers surrounding them.
Finally, they can continue to perform specific movements with which they continue to swim and maintain the body heat they need to survive.
This adaptation is called unihemispheric sleep, in which some animals sleep with only one hemisphere of the brain while the other remains awake.
While sleeping, the dolphin turns off half of its brain, going into what we know as the alpha sleep state (relaxed state), with simultaneous closing of just one eye.
The slow-wave sleep allows them to stay alert to danger even when in periods of inactivity.
On the other hand, the other side of the brain remains awake, in a low state of alert, so that there is the monitoring of predators, obstacles, and other animals, as well as signaling when to emerge to breathe.
After about 2 hours, the animal reverses this process, resting the previously active part of the brain and waking up the rested part.
Some dolphins do not sleep in the first month of their lives. Instead, they rest by pressing their bodies against their mothers to prevent drowning.
Thus, females stay awake because they have to control their babies and stay up for at least a month. In captivity, dolphins can enter a state of complete sleep while the reflex action of the tail keeps their nostrils above the water.
8. Shark- Sleep After All
| Scientific Name | Selachimorpha |
| Phylum | Chordata |
| Sleep Duration | Rest for a short period |
| Sleep During | Day and Night |
| Unique Feature | Sleep with one eye open and Rest with one-half of their brain active |
For many years, scientists believed that sharks don’t sleep at all. But Australian scientists have refuted these assumptions and confirmed that sharks still sleep, like people, reports ScienceAlert.
“As part of our study, we found evidence that sharks sleep, although many believed this is not the case,” says Michael Kelly from Western Australia University.
A team of scientists involved observed the draughtsboard shark being active at night. The study showed that stronger electrical stimulation is needed to make this animal respond.
During these resting periods, the sharks lowered their metabolic rate. The scientists also discovered that 38% of sharks kept their eyes open while resting. Mostly the shark keeps their bodies in a horizontal position- a signal they are sleeping.
In 2016, scientists from the Woods Hole Oceanographic Institution and the Mexican conservation organization Pelagios Kakunja released a white shark napping video.
During the day, this shark stays near the surface and seals colonies, looking for prey and hunting. However, the female sinks to the depths at night and swims calmly against the current near the bottom.
This current, which travels around three kilometers per hour, ensures fresh water constantly washes around the gills without the shark making any particular effort.
Unlike during the day, the fish does not remain stationary at night but circles parts of the island. At the same time, it keeps its mouth open and seems to be in a trance.
In this state, the sharks hardly reacted to external stimuli such as approaching divers, although their sensory organs were fully active.
9. Ant- Hard Work Need Frequent Sleep
| Scientific Name | Formicidae |
| Phylum | Arthropod |
| Sleep Duration | 4 to 5 hours |
| Sleep During | Anytime |
| Unique Feature | Sleep for a minute or 2 |
Many scientific studies stated that this insect does not sleep at all. However, recent tests conducted by American scientists disproved this hypothesis since every living being needs periodic rest.
And the ant is no exception. Scientists found that ants sleep several times a day, and its duration usually does not exceed two minutes.
The sleep nature is recovery sleep as the initial sleep is never enough for the hard-working insects.
They sleep with their heads and antennae down, but since living in the anthill does not stop for a second, they have to doze for about two minutes, up to two hundred times a day.
Since ants sleep for a minimum period, but many times a day, sleep can catch a representative of the colony anywhere. During rest, ants tilt their heads and lower their antennae.
After a few seconds, the individual wakes up, comes to their senses with rapid eye movement, and then continues their march.
The queens sleep similarly. However, after conducting round-the-clock monitoring of the colony’s life, the researchers discovered a peculiarity: if the queen sleeps with her head slightly raised, this indicates superficial sleep.
If she sleeps with her antennae and heads down, we are talking about a deep sleep that lasts longer than usual.
During the winter, ants living in areas with moderate or harsh climatic conditions can fall into a state of suspended animation, which can also be described as a long sleep.
If a sleeping Ant is touched by relatives running past, this individual has no response. It can also occur when there is a threat to the anthill, and the relatives disturb the sleeping individuals, reporting the need to protect the colony.
10. Alpine Swift- Sleeps in the air
| Scientific Name | Tachymarptis melba |
| Phylum | Chordata |
| Sleep Duration | Sleep for a short period |
| Sleep During | Flight in air |
| Unique Feature | Sleep for a minute or 2 |
Swifts and other diurnal creatures possess mammalian-like homeostatic regulation. The slow wave activity (SWA) is at its peak at the beginning of the night and gradually falls down as time is spent.
However, consciousness during radip eye movement REM sleep and non-REM sleep is limited for both avians and mammals due to limited brain functionality during sleep homeostasis.
In 2013, researchers from the Swiss ornithological station proved that this remarkable bird could stay in the air for more than six months and acquire a sleep-like state.
According to the journal “Nature Communications,” six alpine swifts weighing 80-120 g were equipped with geolocators. Three birds were also equipped with an activity sensor. The sensor recorded whether the bird flaps its wings or not.
According to the study of the retrieved data, Alpine swifts slept at least to some extent in flight and did not suffer from sleep deprivation. Like some marine animals, the Alpine Swift can sleep with one side of the brain while the other focuses on flying and identifying.
The super-fast little bird can therefore maintain its bodily functions even during long-term flights. Flying is significantly more energy-intensive than running or swimming. Scientists have assumed that all birds have to sleep to recover their brain functions.
11. Fruit Flies: Can survive without any type of sleep
| Scientific Name | Drosophila melanogaster |
| Phylum | Arthropod |
| Sleep Duration | 16 hours of potential sleep and rest cycle |
| Sleep During | Night on Non ambient temperatures |
| Unique Feature | Survive without sleep but can lower body activity levels |
Fruit flies show a normal sleep behavior of activities during the day and sleeping at night. Surprising studies by Imperial College London revealed that sleep is unnecessary for these simple creatures to live.
This doesn’t mean that they are okay with insufficient sleep. They must sleep for at least 16 hours for their daily functions.
The studies on fruit flies show that lack of restful sleep or appropriate periods of rest can affect the network of nerve cells.
The effects of sleep deprivation hamper body functions and the range of behaviors in fruit flies.
What exactly happens if fruit flies do not sleep? The answers to questions are they die. The new studies by animal behavior scientist Gilestro found that the fact is false.
Lowering of body activity due to inadequate rest at the cellular level decreases the lifespan of fruit flies as of normal house flies.
Every creature on this planet needs sleep; we humans need sleep to consolidate memories and eliminate unnecessary ones. However, animals are not lucky though. They must be constantly alert, even when sleeping, to protect against predators.
No animals have yet been observed that survive without sleep. The period of rest may vary, but every animal takes a break and rests now and then.
(Last Updated on August 2, 2022 by Sadrish Dabadi)
