Miya Pearson
Sleep is essential for human health and survival, but do all living organisms need it? Sleep is vaguely defined as a state of immobility that can be quickly reversed, combined with a reduced rate of responsiveness to stimuli. Scientists are unsure whether every species needs sleep, especially as it is challenging to define what sleep is. Research has shown that some animals never exhibit a state that meets the behavioural definition of sleep. However, sleep is prevalent among animals with nervous systems, and some species will die from sleep deprivation.
| Characteristics | Values |
|---|---|
| Purpose of sleep | Restoration and detoxification, improved cognitive performance, energy conservation |
| Sleep definition | A state of immobility that can quickly be reversed, combined with a greatly reduced rate of responsiveness to stimuli |
| Sleep in animals | All animals with a nervous system seem to sleep, but it is difficult to define sleep in reptiles, fish, and invertebrates |
| Sleep in invertebrates | Research in flies, zebrafish, and nematode worms suggests they sleep, but it is hard to define |
| Sleep in marine mammals | Unihemispheric sleep, where one half of the brain can rest while the other half remains alert |
| Sleep deprivation | Lethal in rats, causes weight loss, impaired immune system, and infections |
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What You'll Learn
Sleep is essential for human health
Firstly, sleep allows our bodies to recharge and repair. When we sleep, our body temperature and heart rate decrease, and our brainwaves slow down, allowing our bodies to restore and replenish energy for the next day. This is particularly important for infants and young children, who need more sleep to support their growth and development.
Secondly, sleep plays a crucial role in memory consolidation and cognitive performance. During sleep, our brains process and reinforce memories, helping us retain information and improve our learning abilities. Sleep deprivation, on the other hand, impairs our memory, cognitive abilities, and reaction times, making it difficult to focus and make decisions.
Thirdly, sleep is vital for maintaining physical and mental health. Lack of sleep can lead to mood swings, impaired judgment, and even hallucinations. It also weakens our immune system, making us more susceptible to illnesses and infections. In extreme cases, prolonged sleep deprivation can lead to serious health issues and even death.
Additionally, sleep helps regulate our body's internal clock, or circadian rhythm. This 24-hour cycle regulates sleep and wakefulness, influencing various biological processes such as hormone production and body temperature. Disruptions to this cycle, such as those caused by shift work or jet lag, can have negative consequences for our health and well-being.
While the reasons why we need sleep are not yet fully understood, it is clear that sleep is essential for maintaining physical and mental health, enhancing cognitive performance, and supporting growth and development. Getting adequate, quality sleep is a critical component of a healthy lifestyle.
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Sleep deprivation in humans and animals
Sleep is a basic human need, just like eating, drinking, and breathing. However, not all animals exhibit a state that meets the behavioural definition of sleep. Sleep deprivation in humans can lead to impaired memory and reduced cognitive abilities, mood swings, and even hallucinations. It can also cause serious physical and mental health issues, including heart disease, kidney disease, high blood pressure, diabetes, stroke, obesity, and depression.
In animals, the effects of sleep deprivation vary across different species. Rats completely deprived of sleep die within a few weeks, experiencing weight loss, infections, and an inability to regulate body temperature. Sleep-deprived birds may show reduced accuracy and performance, while sleep-deprived fish may exhibit decreased responsiveness to stimuli.
In general, acute short-term sleep loss may not have significant adverse health effects on humans, but the health effects of chronic disruption of sleep are increasingly being recognized.
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Differences in sleep habits and needs
The amount of sleep needed varies greatly across species. Human newborns require up to 19 hours of sleep in a 24-hour period, while adult humans need at least 7 hours of sleep per night. In comparison, some animals require much more sleep. For example, three-toed sloths need nearly 16 hours of sleep a day, and two-toed sloths need 16.4 hours. Other long sleepers include the little brown bat (19.9 hours), the North American opossum (19.4 hours), and the giant armadillo (18.1 hours).
On the other hand, some large land mammals require very little sleep. African elephants sleep an average of two hours a day, and cows and horses sleep between three and four hours a day.
Not only does the required amount of sleep vary among humans and other animals, but sleep cycles and processes that take place during sleep also differ. These differences in sleep habits and needs are caused by several factors, including brain size, diet, body mass index (BMI), and social hierarchy. For example, predatory animals usually sleep in longer uninterrupted periods, and they are typically diurnal, sleeping at night like humans, or nocturnal, sleeping during the daytime, like tigers.
REM (rapid-eye movement) sleep also varies across species. Humans experience REM sleep approximately every 90–120 minutes during sleep, while mice experience it every 10–15 minutes. Some animals, such as dolphins and whales, do not show typical behaviours associated with REM sleep, although whales do exhibit some muscle jerking that could be representative of REM sleep.
In addition to the amount of REM sleep varying by species, so too does the function of sleep. One theory is that it enables restoration and detoxification, producing molecules and removing waste products. Another theory is that sleep improves cognitive performance by providing a period for the brain to reinforce or prune synaptic connections among its neurons, aiding learning and memory without interference from fresh sensory information. A third theory suggests that sleep saves energy, balancing the need to find food with spending energy foraging. This is shaped by the ecological niche of the animal—small mammals may have only a short window to hunt insects, while large herbivores gain few calories from vegetation. This may explain why the little brown bat sleeps for 20 hours a day, while an African bush elephant sleeps for just two hours in the wild.
The way in which animals obtain their sleep and rest also differs. Unihemispheric or half-brain sleep is found in whales, dolphins, fur seals, and great frigatebirds, among others. These animals keep one eye open to watch for predators and potential dangers while swimming or flying, allowing them to stay safe.
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Sleep cycles and processes
The cycles of REM sleep differ across species. For example, humans experience REM sleep approximately every 90–120 minutes during sleep, while mice experience REM sleep every 10–15 minutes.
Some animals, such as whales, dolphins, and fur seals, exhibit unihemispheric sleep, where one half of the brain exhibits sleep characteristics while the other half remains alert. This allows them to swim to the water's surface to breathe or watch for predators while sleeping.
The duration of sleep also varies among species. Human newborns require up to 19 hours of sleep in a 24-hour period, while adult humans need at least 7 hours of nightly sleep. In comparison, some animals require much more sleep. For example, three-toed sloths need nearly 16 hours of sleep a day, while two-toed sloths need 16.4 hours. On the other hand, some large land mammals require very little sleep. African elephants sleep for about two hours a day, while cows and horses sleep between three and four hours.
The way animals divide their sleeping time also differs from humans. Human sleep after early childhood is typically monophasic or biphasic, occurring during one portion of a 24-hour period, with a possible short nap during the day. In contrast, animal sleep is often polyphasic, divided into several shorter periods throughout the day. For instance, dogs tend to sleep between 9 and 14 hours each day but only in 45-minute bouts. Cats sleep up to 13 hours a day in 78-minute periods.
Predatory animals usually sleep in longer, uninterrupted periods, either diurnally (at night) or nocturnally (during the day).
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Theories on why animals need sleep
The function of sleep is one of biology's biggest mysteries. While sleeping, an animal is less alert to its environment and more vulnerable to predators. However, all animals with a nervous system seem to engage in some form of sleep, meaning the benefits must outweigh the costs.
There are three general theories as to why animals need sleep, two of which involve the brain. The first theory is that shutdown enables restoration and detoxification, producing molecules and removing waste products. The second is that sleep improves cognitive performance by providing a period in which the brain can reinforce or prune synaptic connections among its neurones – to help with learning and memory without being confused by fresh sensory information.
The third theory, not involving the brain, is that sleep saves energy. Animals must balance a need to find food with spending energy to forage. This is shaped by their ecological niche – small mammals may have only a short window to hunt insects, whereas large herbivores gain few calories from vegetation.
The three theories for sleep’s function aren’t mutually exclusive, and all have weaknesses. Even brainless jellyfish sleep, and you’d expect intelligent elephants – who famously never forget – to sleep longer.
Sleep can be defined physiologically or behaviourally. Physiological sleep is characterised by reversible unconsciousness, special brainwave patterns, sporadic eye movement, loss of muscle tone and a compensatory increase following deprivation of sleep, known as sleep homeostasis. Behaviourally, sleep is characterised by minimal movement, non-responsiveness to external stimuli, the adoption of a typical posture, and the occupation of a sheltered site, all of which is usually repeated on a 24-hour basis.
Sleep as a phenomenon appears to have very old evolutionary roots. It is observed in mammals, birds, reptiles, amphibians, fish, and, in some form, in insects. The internal circadian clock promotes sleep at night for diurnal organisms (such as humans) and in the day for nocturnal organisms (such as rats). Sleep patterns vary widely among species, with some foregoing sleep for extended periods and some engaging in unihemispheric sleep, in which one brain hemisphere sleeps while the other remains awake.
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Frequently asked questions
While it is not known for sure if all living organisms need sleep, it is understood that all animals with a nervous system engage in some form of sleep. Sleep is defined as a state of immobility that can quickly be reversed, combined with a greatly reduced rate of responsiveness to stimuli.
No, the amount of sleep animals need varies greatly across species. While human newborns require up to 19 hours of sleep in a 24-hour period, and adult humans require at least 7 hours of nightly sleep, many animals require much more sleep. For example, three-toed sloths need nearly 16 hours of sleep a day, and two-toed sloths need 16.4 hours. On the other hand, some large land mammals require very little sleep. African elephants sleep an average of two hours a day, and cows and horses sleep between three and four hours a day.
REM stands for rapid-eye movement sleep. It is characterized by quick and chaotic brainwaves, eye twitches, and muscle twitching. REM sleep is also when humans dream. While many terrestrial mammals, some reptiles, birds, and aquatic invertebrates experience REM sleep, some animals, such as dolphins and whales, do not show typical behaviors associated with it.
