You’ve probably heard that adult humans should sleep an average of 8 hours a night to properly fulfil their (or “our”) daily personal, professional and academic obligations. Getting enough rest is extremely important, as it brings a lot of physiological (and adaptive) benefits and advantages. But resting is not enough; we also need to get good quality sleep, which will improve the effectiveness of our actions.

In the case of humans, these recommendations are drawn from theory, which can only be applied in ideal conditions. However, taking theory to practice when circumstances are not ideal or constant does not guarantee results – and this is what happens in nature. Not all mammals need the same amount of sleep, as they don’t share the same needs. Therefore, their sleep time will depend on a number of factors, such as diet or the way of life of each species. In this article, we’re going to explain how sleep mechanisms work, their impact on the daily life of species and we’ll discover what mammals spend more hours sleeping. But first…

The ring-tailed lemur (Lemur catta) can sleep up to 16 hours a day. By David Havel |
The ring-tailed lemur (Lemur catta) can sleep up to 16 hours a day. By David Havel |

What are sleep cycles?

In simple terms, sleep can be described as a state in which the body is extremely inactive; it’s both habitual and adaptive, and is linked to specific positions, places and neural states. This period of inactivity comes with important physiological benefits, and also allows to save energy and reduce exposure to different types of risks. Additionally, during deep sleep, there is a reduction in the monitoring of the environment. An example of this is the sleep cycle of dolphins: every 2 hours, one of their brain hemispheres sleeps, while the other stays awake and keeps an eye out for potential risks.

During sleep, dolphins reduce the activity of one brain hemisphere, while the other remains active. By Christopher Berthelot |
During sleep, dolphins reduce the activity of one brain hemisphere, while the other remains active. By Christopher Berthelot |

But what are the main functions of sleep? Several have been proposed, including a recovery function and a protective function. The former allows the body to recharge or reorganize its physiological or neurological processes, such as the release of hormones during sleep. The growth hormone is particularly important here, as it isn’t produced during the day and its maximum release peak is 50-60 minutes into sleep. Melatonin is also critical, as it regulates sleep and wakefulness, and is produced at sunset.

Regarding the protective function of sleep, remaining still and inactive under unfavourable conditions (both during day and night) reduces the risk of predation and of mechanical damage, and also allows to save energy. Another theory is that sleep has no specific function and that its origin is purely “historical”, a result of day-night periodicity. In other words, sleep is a rhythm.

Daily biological rhythms

Biological rhythms are a sequence of events that repeat regularly over time, that have physical characteristics (period, frequency and amplitude) and can affect the biological variables at any level of organisation. Most recognised rhythms last approximately a day (22 to 28 hours) and are called ‘circadian rhythms‘. They regulate the sleep and wake cycle, but also affect other biological variables, such as body temperature or heart rate.

Circadian rhythms are considered endogenous, as they are generated by the organism itself and not by external or geophysical factors (though they are often synchronous with external events). Endogenous rhythms affect a number of ecological and physiological processes (such as hormone levels, locomotion or the accumulation of fats) and can change with environmental circumstances. For example, they depend on whether animals are wild or domestic, or on whether they are bred in captivity.

Sloths can sleep up to 20 hours a day, but this can vary if the environmental circumstances change. By Kleber Varejão Filho
Sloths can sleep up to 20 hours a day, but this can vary if the environmental circumstances change. By Kleber Varejão Filho

Circadian rhythms are also adaptive and are examples of predictive homeostasis that allow animals to anticipate changes. For example, sleep onset takes place 5 to 6 hours before body temperature hits its lowest point (between 2 and 4 a.m.).

The biological clocks that regulate the organism

Biological clocks are responsible for coordinating a range of biological functions, such as body temperature or sleep cycles. Serotonin, for example, is a neurotransmitter that can act as an internal clock that regulates the sleep and wake cycles. When it coordinates all the different biological variables, we enjoy a deep, refreshing sleep and wake up well rested.

In order to determine the existence of a biological clock in different mammals, it’s enough to find out whether their rhythms persist when they’re moved to other parts of the world (which is what happens when we experience jet lag) or if the endogenous rhythms deviate from the natural rhythm under constant laboratory conditions. If a species has a biological clock, its rhythms can show a frequency that is not synchronous with external periodic factors.

The REM phase or the ‘paradoxical sleep’

In mammals, the sleep period consists of several cycles – each with four states – that are separated by the REM (rapid eye movements) phase, which usually marks the end of each of them. During the REM phase, the right nasal hole breathes in more air, which means that the left hemisphere of the brain is more active (this also happens during the most active times of the day). During the REM phase of sleep, the brain of mammals behaves similarly to when they’re awake. This phase is probably a result of the brain’s interpretation of the different activities and signs. Additionally, it “tunes up” and activates the organism, which means it has adaptive value.

The different phases of sleep and REM sleep
The different phases of sleep and REM sleep

In addition, during the REM phase (also known as the ‘paradoxical sleep’ and that usually lasts around 90-120 minutes a night), the brain sends contradictory messages to the body, such as moving and staying still, which are not fully neutralised. Although the brain shows activity, the muscles stay relaxed, as the release of certain neurotransmitters is inhibited (a process called ‘adaptive non-responding’).

The sleepiest mammals

SpeciesHours of sleepSpeciesHours of sleep
Sloth / Bat20Hedgehog /Chimpanzee9
Armadillo19Human / Mole8
Koala18Guinea pig / Cow7
Lemur / Tupaia16Tapir / Sheep6
Hamster / Squirrel14Horse Long-finned pilot whale5
Mouse / Rat / Cat13Okapi / Common bottlenose dolphin 5
Chinchilla / Echidna12Giraffe / Elephant4
Jaguar11Dall’s porpoise / Shrew0

(Summary table with the sleeping hours of different animal species)

We can conclude that predators can afford to sleep longer (carnivores sleep more than herbivores, as they are at lower risk), while small animals also sleep longer hours than larger species.

In general, folivores (animals that eat leaves) are the group that sleeps more hours in total, while animals that need to feed constantly, such as shrews and elephants, get very little sleep. In fact, due to their high weight, they can’t spend long periods of time lying down.

Translated by Carlos Heras


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