Miya Pearson
Sleep is a biological requirement for nearly all animals, including mammals. However, the amount of sleep a mammal needs depends on several factors, such as age, body size, environment, diet, and the safety of its sleep site. Some mammals, like bats, sleep for 18-20 hours a day, while others, like giraffes and elephants, sleep for only 3-4 hours. Sleep patterns also vary, with some mammals sleeping in multiple short periods throughout a 24-hour cycle and others sleeping for longer, concentrated periods. All mammals studied thus far exhibit signs of REM sleep, suggesting that they dream, just like humans.
| Characteristics | Values |
|---|---|
| Purpose of sleep | Restoration and detoxification, cognitive performance, energy conservation |
| Sleep patterns | Monophasic (one concentrated time period) or polyphasic (multiple periods throughout a 24-hour cycle) |
| Sleep duration | Depends on factors such as age, body size, environment, diet, and safety of sleep site |
| Sleep deprivation | Can lead to weight loss, reduced body temperature, infections, and death |
| REM sleep | All mammals studied thus far exhibit REM sleep, suggesting they dream |
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What You'll Learn
Do mammals need REM sleep?
Sleep is a biological requirement for almost all animals, except for those with no brain or only a rudimentary brain. Sleep patterns vary widely among species, with some foregoing sleep for extended periods and others engaging in unihemispheric sleep, where one brain hemisphere sleeps while the other remains awake.
Sleep can be defined physiologically or behaviourally. In the physiological sense, sleep is a state characterised by reversible unconsciousness, special brainwave patterns, sporadic eye movement, loss of muscle tone, and a compensatory increase following deprivation of the state, known as sleep homeostasis. In the behavioural sense, 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.
REM sleep is one of the two types of sleep identified in mammals, the other being non-REM sleep (slow-wave sleep). REM sleep is characterised by a pattern of brain activity that closely resembles that of waking in most brain regions. Brainstem neurons are highly active, often at rates equal to or exceeding those in active waking. Cortical neurons also show a waking pattern of activity, with the electroencephalogram (EEG) in many species being indistinguishable from that of waking.
REM sleep has been observed in all terrestrial mammals studied thus far, suggesting that mammals dream just like humans do. However, some marine mammal species do not show evidence of REM sleep, and convincing evidence for this state in reptiles, fish, and insects is lacking.
REM sleep in mammals can be accompanied by either high- or low-voltage cortical activity. Conversely, waking is usually characterised by low-voltage cortical activity, but it can be accompanied by a high-voltage EEG similar to that seen in non-REM sleep. Despite the similarity of REM sleep brain activity to the waking pattern, the loss of environmental awareness that characterises non-REM sleep is also present in REM sleep. Although most neurons behave similarly in REM sleep and waking, certain neurons that are tonically active throughout waking states are silent in REM sleep. The reduction in activity of these neurons has been linked to the reduction in muscle tone that occurs in REM sleep, and the loss of consciousness occurring in sleep.
The presence of REM sleep in all terrestrial mammals studied so far indicates that it is likely necessary for the survival of mammals. Rats deprived of sleep die within a few weeks. Sleep-deprived rats lose weight despite increasing food intake, progressively fail to regulate body temperature, and develop infections, suggesting an impairment of the immune system.
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What are the risks of not sleeping?
Sleep is essential for all mammals, and a lack of sleep can have severe consequences for their health and survival. Here are some of the risks associated with sleep deprivation in mammals:
- Weight Loss and Appetite Changes: Sleep-deprived mammals may experience a decrease in appetite, which can lead to rapid weight loss. This was observed in rats, where despite increasing their food intake, they lost weight and eventually died.
- Immune System Dysfunction: Sleep deprivation can impair the immune system, making mammals more susceptible to infections and diseases. Sleep-deprived rats, for example, developed infections and fatal sepsis.
- Increased Risk of Accidents: Sleep deprivation can impair judgment and reaction time, increasing the risk of accidents. This is particularly dangerous for mammals that need to stay alert and agile to avoid predators or navigate their environment safely.
- Cognitive Impairment: Lack of sleep can lead to impaired memory, reduced cognitive abilities, and mood swings. Prolonged sleep deprivation can even induce hallucinations.
- Physiological Changes: Sleep helps regulate body temperature and metabolism. Sleep-deprived mammals may experience changes in body temperature, increased heart rate, and altered electrical activity in the brain.
- Increased Mortality: In extreme cases, prolonged sleep deprivation can lead to death. Rats deprived of sleep died within a few weeks, indicating the critical importance of sleep for survival.
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How does sleep duration vary among mammals?
Sleep duration varies significantly among mammals. The amount of sleep a mammal needs depends on several factors, including age, body size, environment, diet, and the safety of its sleep site. 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, some animals sleep much more. The little brown bat sleeps for about 20 hours a day, while the North American opossum sleeps for 19.4 hours, and the giant armadillo for 18.1 hours.
On the other hand, some large land mammals require very little sleep. African elephants sleep for an average of just two hours a day, while cows and horses sleep between three and four hours. The average giraffe sleeps for about 4.6 hours per day, with short sleep cycles of 35 minutes or less.
The difference in sleep duration between species is related to their feeding habits. Carnivores have the highest daily need for sleep, followed by omnivores, and then herbivores. For example, lions, with little fear of predators, have relatively long sleep periods, while elephants have to eat most of the time to support their huge bodies. Little brown bats conserve their energy, except for a few hours each night when their insect prey are available.
The way mammals sleep also differs. Monophasic sleep describes animals that generally receive their sleep in one concentrated time period, like humans. Polyphasic sleep is more common, with animals sleeping in multiple periods throughout a 24-hour cycle, as they need to maintain some level of vigilance against predators. However, if threats are minimized, animals can enjoy monophasic sleep. For example, marmosets sleep in trees surrounded by their family, enabling them to feel more protected and experience monophasic sleep.
Some mammals, such as horses and giraffes, can sleep standing up, but they lie down for REM sleep. Walruses can sleep in water and on land, although they sleep for longer periods on land. Dolphins, eared seals, and manatees are all marine mammals that sleep unihemispherically, with one side of the brain sleeping while the other stays awake.
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How does the environment affect sleep?
The environment can have a significant impact on sleep quality and duration. Environmental factors such as temperature, noise, light, bed comfort, and electronic distractions can all influence an individual's ability to get a good night's sleep.
Noise
Excessive noise during sleep can decrease sleep intensity, cause frequent awakenings, and increase stress hormone secretion. Intermittent sounds, such as honking or revving cars, are more disruptive to sleep than continuous noise. A study in Germany found that road traffic noise had the strongest association with objective sleep measures, while subjective sleep quality was most strongly associated with air and rail traffic noise exposure.
Light
Artificial light exposure at night can delay sleep onset and disrupt circadian rhythms by suppressing melatonin secretion. A study of the US population found that those with greater outdoor nighttime light exposure had a 28% greater chance of experiencing a circadian phase delay and a later bedtime.
Temperature
Finding a comfortable temperature for sleeping is essential. While a cooler temperature is often recommended for optimal sleep, finding the temperature that suits an individual's preference is crucial.
Electronic Distractions
The presence of electronic devices, such as televisions, computers, and cell phones in the bedroom, can negatively impact sleep. A study of Israeli adolescents found that those with excessive electronic media habits experienced more daytime sleep-related problems and went to bed later than those without such devices in their bedrooms.
Safety and Social Factors
Living in communities with greater social cohesion and a sense of safety is associated with better sleep health. Conversely, living in unsafe neighborhoods can impair an individual's ability to relax and fall asleep.
Neighborhood Disadvantage
Neighborhood disadvantage, such as high poverty rates, female-headed households, unemployment, and low homeownership, has been linked to restless sleep and longer wakefulness after sleep onset. However, the association between neighborhood disadvantage and sleep duration, quality, or latency is inconsistent across studies.
Air Pollution
Exposure to air pollution, such as fine particulate matter and nitrogen oxides, has been associated with a greater risk of moderate to severe sleep apnea. Living near roadways can lead to increased traffic-related air pollution and sleep apnea symptoms.
Natural Environment
Access to green spaces and natural environments has been linked to improved sleep health. Observational studies have shown that adults living in neighborhoods with green spaces or natural water features are less likely to experience insufficient sleep.
Built Environment
The built environment, including walkability, street connectivity, and access to recreational facilities, can impact physical activity levels and sleep. Neighborhoods with higher walkability scores, more social destinations, and greater population and intersection density have been associated with shorter sleep duration.
Social Determinants
Racial/ethnic minorities and individuals of lower socioeconomic status often experience a greater burden of air pollution, neighborhood disorder, and lower proximity to green spaces, which can contribute to poorer sleep quality and duration.
Household and Family Context
Family and household factors, such as parental behaviors, family structure, and socioeconomic status, can also influence sleep patterns, especially in children and adolescents. Maternal depression and anxiety have been linked to increased sleep problems in infants, while marital stability has been found to predict better sleep in children.
Interventions
Interventions targeting changes in the environment, such as reducing artificial light, improving aesthetic qualities, and promoting walkability, may help improve sleep outcomes and reduce sleep disparities.
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What are the benefits of sleep?
Sleep is essential for the health of humans and other mammals. The benefits of sleep include:
- Restoration and detoxification: Sleep enables the body to produce molecules and remove waste products.
- Cognitive performance: Sleep provides a period in which the brain can reinforce or prune synaptic connections among its neurons, helping with learning and memory.
- Energy conservation: Sleep helps to conserve energy, although this theory is not entirely adequate as it only decreases metabolism by about 5-10%.
- Immune system: Sleep deprivation in rats has been shown to cause a 20% decrease in white blood cell count, indicating a significant change in the immune system.
- Growth and development: Sleep is necessary for the appropriate growth and development of younger humans and other mammals.
- Memory consolidation: Sleep allows humans to consolidate memories.
The amount of sleep required varies across different species of mammals. For example, African elephants sleep for an average of two hours a day, while little brown bats sleep for about 20 hours a day.
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Frequently asked questions
Yes, all mammals studied so far do need sleep. Sleep is a biological requirement for all animals except for basal species with no brain or only a rudimentary brain.
The amount of sleep a mammal needs depends on several factors, including age, body size, environment, diet, and the safety of its sleep site. For example, African elephants sleep for an average of two hours a day, while little brown bats sleep for about 20 hours a day.
Mammals sleep in different ways. Some sleep standing up, some lying down, and some even hang upside down. Some mammals sleep with one hemisphere of the brain at a time, allowing them to swim or fly while staying vigilant against threats.
Sleep deprivation in mammals can lead to weight loss, reduced body temperature, skin lesions, impaired memory, mood changes, and even death. For example, rats that are sleep-deprived die within a few weeks.
