Animals And Rem Sleep: What Science Says

Sleep is a biological requirement for almost all animals, except for the most basic species with no brain or a very simple brain. All mammals studied so far exhibit signs of REM sleep, suggesting that they dream like humans. However, REM sleep is not limited to mammals; birds, reptiles, amphibians, fish, and even insects experience REM sleep. The amount of REM sleep varies widely depending on the species. While the functions of REM sleep are not yet fully understood, it is believed to be involved in memory consolidation and brain maturation in early life.

Do animals dream?

Sleep is a biological requirement for almost all animals. It is characterised by reversible unconsciousness, special brainwave patterns, sporadic eye movement, loss of muscle tone, and a compensatory increase following deprivation.

The amount of sleep animals need varies across species. For example, human newborns require up to 19 hours of sleep in a 24-hour period, while adult humans require at least 7 hours. In comparison, three-toed sloths need nearly 16 hours of sleep a day, and two-toed sloths need 16.4 hours.

Different mammals spend different amounts of time in non-REM sleep and REM sleep. All mammals studied thus far do exhibit signs of REM sleep, suggesting that mammals dream, just like humans do. However, REM sleep has not been observed in some animals, such as dolphins and whales.

The cycles of REM sleep vary across species, too. Humans experience REM sleep approximately every 90–120 minutes during sleep, while mice experience REM sleep every 10–15 minutes.

The amount of REM sleep also varies widely depending on the species. House cats can spend up to 8 hours a day in REM sleep, whereas elephants don't experience it daily due to their minimal sleep.

Some animals, such as dolphins and whales, exhibit unihemispheric sleep, in which one brain hemisphere sleeps while the other remains awake. This allows them to swim to the water's surface to breathe while sleeping.

Recent studies suggest that REM sleep, or ancestral forms of it, might be found in non-mammalian or non-avian species such as reptiles. These observations suggest that REM sleep and non-REM sleep evolved earlier than previously thought.

In summary, while it is unclear whether animals dream in the same way humans do, the presence of REM sleep in many mammals suggests that they do experience some form of sleep mentation.

What is REM sleep?

REM sleep, or rapid-eye movement sleep, is one of two distinct sleep states that mammals and birds experience. It is characterised by muscle twitching, waking-like electrical brain patterns, and, as the name suggests, rapid eye movements.

During sleep, the body cycles through four stages, with physical and electrical changes occurring during each. The fourth stage is REM sleep, which is preceded by non-REM sleep. The duration and frequency of REM sleep varies across species. For example, humans experience REM sleep approximately every 90-120 minutes, while mice experience it every 10-15 minutes.

REM sleep is thought to be involved in memory consolidation, and its function may be related to the physiology of non-REM sleep. In humans, it is during REM sleep that we are most likely to dream.

REM sleep has been identified in birds, and there is evidence to suggest that reptiles, the common ancestors of birds and mammals, may also experience it. However, it is not known whether REM sleep is present in all animals.

How does REM sleep differ from non-REM sleep?

REM sleep and non-REM sleep are the two main types of sleep. They differ in several ways, including brain wave activity, muscle tone, breathing, and heart rate.

Brain Wave Activity

REM sleep is characterised by brain activity that resembles wakefulness more closely than non-REM sleep. During REM sleep, the brain is highly active, and brain waves are more variable. In contrast, during non-REM sleep, brain waves slow down.

Muscle Tone

During REM sleep, there is a complete loss of muscle tone, whereas non-REM sleep is associated with partial muscle tone. This loss of muscle tone during REM sleep may serve as a protective measure to prevent people from acting out their dreams, although this hypothesis is questionable since we now know that dreams can occur during non-REM sleep as well.

Breathing

Breathing becomes irregular during REM sleep, while it remains steady and slower during non-REM sleep.

Heart Rate

Heart rate rises during REM sleep, whereas it slows down during non-REM sleep.

Arousal Threshold

It is easier to wake someone from non-REM sleep than from REM sleep. Waking someone from deep sleep, or non-REM sleep, can result in sleep inertia, a state of confusion or "mental fog" that lasts about 30 minutes.

Occurrence

After falling asleep, people typically enter non-REM sleep before transitioning to REM sleep. Throughout the night, the body alternates between non-REM and REM sleep in cycles. The first cycle of REM sleep is usually the shortest, lasting around 10 minutes, while subsequent cycles become progressively longer, up to an hour.

Why do animals need sleep?

Sleep is a biological requirement for nearly all animals, except for the most basal species with no brain or only a rudimentary brain. Animals need sleep to restore their bodies and minds, and to feel rested.

The amount of sleep animals need varies greatly across species. For example, human newborns require up to 19 hours of sleep in a 24-hour period, while adult humans require at least 7 hours of nightly sleep. In comparison, many animals require much more sleep. Sloths need nearly 16 hours of sleep a day, while the little brown bat sleeps for 19.9 hours. On the other hand, some large land mammals require very little sleep. African elephants sleep for an average of two hours a day, while cows and horses sleep between three and four hours a day.

The amount of sleep a mammal needs depends on several factors, including age, body size, environment, diet, and the safety of its sleep site. Whether a mammal lives on land or in the sea can also affect how much sleep it needs. For example, marine mammals like dolphins, whales, and pinnipeds (seals) engage in unihemispheric sleep, where one brain hemisphere sleeps while the other remains awake, allowing them to swim to the water's surface to breathe while sleeping.

Sleep can be defined physiologically or behaviorally. Physiological sleep is characterised by reversible unconsciousness, special brainwave patterns, sporadic eye movement, loss of muscle tone, and a compensatory increase following deprivation. Behavioural sleep is characterised by minimal movement, non-responsiveness to external stimuli, the adoption of a typical posture, and occupation of a sheltered site, all of which is usually repeated on a 24-hour basis. The physiological definition applies well to birds and mammals, but the behavioural definition is more often used for other animals with less complex brains.

The internal circadian clock promotes sleep at night for diurnal organisms (such as humans) and during the day for nocturnal organisms (such as rats). Sleep patterns vary widely among species, with some foregoing sleep for extended periods. Sleep deprivation has serious consequences for complex animals, and no animal species has been found that does not need recovery sleep.

REM (rapid-eye movement) sleep has been identified in birds, but very few avian species have been investigated. REM sleep has also been observed in reptiles, and recent studies suggest that REM sleep, or ancestral forms of it, might be found in non-mammalian or non-avian species. These observations suggest that REM sleep and non-REM sleep evolved earlier than previously thought.

The presence of dreams in human sleep, especially in REM sleep, and the detection of similar states in mammals, has led many to wonder if animals experience similar sleep mentation. Evidence that dream mentation occurs during both non-REM and REM sleep indicates that all mammals have the potential to experience dream mentation. However, this non-REM dream mentation may be different in species where non-REM sleep is atypical, such as during unihemispheric sleep in aquatic mammals.

How much sleep do animals need?

The amount of sleep an animal needs depends on a variety of factors, including age, body size, environment, diet, and the safety of its sleep site. Whether an animal is a land or marine dweller can also affect how much sleep it requires.

Generally, larger animals need less sleep than smaller ones. For example, giraffes tend to sleep for about 4.6 hours per day, while elephants sleep for about 2–4 hours per day. In contrast, sloths sleep for up to 20 hours per day, and koalas sleep for about 20–22 hours per day.

The feeding habits of animals also play a role in how much sleep they need. Carnivores tend to sleep more than herbivores, as herbivores have to spend many hours eating every day, whereas carnivores can take in a large number of calories in one meal. For instance, lions sleep for about 19 hours per day, while giraffes and elephants sleep for significantly less.

Some animals, such as lions, sleep in short periods during the day and at night, allowing them to take advantage of feeding opportunities. The need to constantly search for food may also reduce the amount of sleep certain animals get. For example, walruses can go without sleep for 84 hours, and elephants may go without sleep for nearly two days while travelling.

The environment and safety of an animal's sleep site also influence its sleep patterns. Animals that feel more protected in their sleeping environment may experience monophasic sleep, where they receive their sleep in one concentrated time period. Marmosets, for instance, sleep in trees surrounded by their family, enabling them to feel safer and experience monophasic sleep. On the other hand, animals that need to maintain some level of vigilance against predators tend to have polyphasic sleep, where they sleep in multiple periods throughout a 24-hour cycle.

In addition, the brain size of an animal can determine how much sleep it needs. Animals with larger brains require more REM sleep, as REM sleep helps the brain consolidate memories and learn.

Overall, the amount of sleep an animal needs varies greatly across species, and is influenced by a multitude of factors.

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