Herbie Murray
Sleep is a universal function among vertebrates, but the number of hours spent sleeping varies across species. While a donkey may sleep for just three hours a day, a hairy armadillo can sleep for over 20 hours. The amount of sleep an animal needs is related to the size of its brain relative to its body. For example, a study found that the ratio of neuron density to surface area in the brain was the best predictor of sleep time across 24 mammalian species. This is because the brain needs to clear metabolites – by-products of daily physiological processes – from its neurons, and a higher ratio of neuron density to surface area means that metabolites accumulate faster. However, other factors such as metabolic rate and evolutionary relationships also play a role in sleep patterns.
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What You'll Learn
The relationship between brain size and sleep is unclear
Some researchers have hypothesized that the time spent sleeping is related to differences in neuron density in the brain and the brain's surface area across species. This hypothesis suggests that larger brains (with more surface area for metabolite rinsing) and less densely packed neurons (allowing metabolites to be rinsed away more readily) are linked to less time sleeping. However, this hypothesis does not explain cases where large-brained animals like cattle only sleep for a few hours each day, while primates with similar-sized brains sleep for up to 8 hours.
To address this discrepancy, it has been proposed that it is the ratio between neuron density and the surface area of the brain that is rinsed with spinal fluid during waking hours that determines the amount of time spent sleeping each day. This ratio, known as the D/A ratio, has been found to positively correlate with sleep time across 24 mammalian species. Specifically, as the ratio between neuron density and surface area increased, so did the amount of time spent sleeping each day.
However, other studies have found conflicting results. One study that analyzed sleep data from 19 mammalian species found a positive correlation between total sleep time and body size, but no significant correlation between sleep time and the D/A ratio after accounting for the effects of body size. Additionally, the relationship between brain size and sleep may be further complicated by other factors such as metabolic rate, ecological factors, and evolutionary relationships. For example, species at risk of predation tend to sleep less, possibly because they cannot afford to sleep for longer periods.
In summary, while some evidence suggests a relationship between brain size and sleep, the specific nature of this relationship remains unclear. Further research is needed to disentangle the complex web of relationships between brain size, body size, neuron density, metabolic rate, ecological factors, and sleep.
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Sleep is important for brain function and cognition
During sleep, the brain's synapses – the connections among neurons – shrink by nearly 20%, resting and preparing for the next day. Without this reset, known as "synaptic homeostasis", synapses could become overloaded and burn out. Sleep is also when the brain consolidates memories, with a good night's sleep improving people's ability to remember what they have learned during the day by about 15%.
Sleep also helps the body to clear metabolites – by-products of our body's daily physiological processes. These metabolites accumulate during the day, and while some are toxic, others – like adenosine – signal a need for sleep. These metabolites are washed away during sleep by cerebral spinal fluid, the liquid that coats our nervous systems. However, during waking hours, the space between cells and neurons in our brains contracts, so the fluid can only clear metabolites at the surface of the brain and not those further below.
The amount of sleep required is also linked to the ratio of neuron density to the surface area of the brain that is rinsed with spinal fluid during waking hours. This is why, for example, giraffes sleep for as little as 3 hours per day, while many primates sleep for 8 hours.
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Sleep may be necessary for the consolidation of memories
Sleep is necessary for the consolidation of memories. It is associated with distinctive patterns of neural activity, which differ between rapid-eye movement (REM) sleep and non-REM sleep. During the REM phase, the brain is highly active and plays a key role in intelligence and cognitive function.
Studies have shown that REM sleep is important for cognitive abilities such as memory consolidation. A good night's sleep, with plenty of REM sleep, can improve people's ability to remember what they have learned during the day by about 15%. Sleep is thought to play a role in synaptic remodelling, and sleep deprivation negatively impacts learning, reaction times, and attention.
Research has also shown that sleep provides a time when the brain's synapses – the connections among neurons – shrink back by nearly 20%. During this time, the synapses rest and prepare for the next day, when they will grow stronger while receiving new input. Without this reset, known as "synaptic homeostasis," synapses could become overloaded and burned out, like an electrical outlet with too many appliances plugged into it.
The amount of sleep required is also linked to the density of neurons in the brain. Dr Suzana Herculano-Houzel of the Federal University of Rio de Janeiro, Brazil, hypothesized that the time spent sleeping might be related to differences in neuron density in the brain and the brain's surface area across species. She found that as the ratio between neuron density and surface area increased, so did the amount of time spent sleeping each day. This suggests that sleep is necessary to clear metabolites – by-products of our body's daily physiological processes – from our brains.
Therefore, it can be concluded that sleep plays a crucial role in memory consolidation and cognitive function, and the amount of sleep required may be influenced by the density of neurons in the brain.
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Sleep is linked to the brain's ability to clear metabolites
However, the brain's ability to clear metabolites is impacted by neuron density and the brain's surface area. During waking hours, the space between cells and neurons in our brains contracts, so the fluid can only clear metabolites at the surface of the brain and not those further below. Similarly, if the neurons in the brain are densely packed beneath the surface, this makes it harder for the fluid to clear these metabolites.
Research by Dr Suzana Herculano-Houzel of the Federal University of Rio de Janeiro, Brazil, found that the ratio between neuron density and the surface area of the brain that is rinsed with spinal fluid during waking hours determines the amount of time spent sleeping each day. As the ratio between neuron density and surface area increased, so too did the amount of time spent sleeping each day.
Herculano-Houzel's findings suggest a 'self-reinforcing spiral' in the evolution of larger brains and bodies: increased neuron density leads to decreased sleep time, thereby allowing animals more time to look for food to support their larger bodily structures.
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Sleep deprivation has negative impacts on cognitive function
Sleep is essential for the brain to function properly. During sleep, the brain clears away waste products, such as metabolites, that have accumulated throughout the day. These waste products can be toxic, and if they are not cleared away, they can damage neurons. Sleep also allows the brain to rest and prepare for the next day, when it will form new connections and learn new things.
The amount of sleep needed varies across species, and while the reason for this variation is not yet fully understood, it may be related to differences in brain size and neuron density. Studies have found that larger brains and less densely packed neurons are linked to less time spent sleeping. However, this does not explain why some large-brained animals sleep less than primates with similar-sized brains.
One hypothesis suggests that it is the ratio of neuron density to the surface area of the brain that determines sleep duration. This hypothesis is supported by findings that show a strong correlation between sleep time and this ratio.
Sleep deprivation can have negative consequences for both physical and mental health. It can lead to impaired physiological and mental functioning and, in extreme cases, even death. Lack of sleep has been linked to increased feelings of hostility, depression, and anger, particularly in women.
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Frequently asked questions
It depends on the animal. Some studies suggest that animals with bigger brains need more REM sleep, but when evolutionary links are controlled for, this relationship disappears. However, it is important to note that the amount of sleep needed varies across species, with some larger animals sleeping less than smaller ones.
REM stands for rapid eye movement. It is a type of sleep during which the brain is highly active and is believed to play a key role in intelligence and cognitive function.
REM sleep is important for cognitive abilities such as memory consolidation. Bigger brains may need more REM sleep to support these functions.
Yes, metabolic rate and evolutionary relationships can also impact sleep patterns. Animals with high metabolic rates may sleep less because they burn more calories and need to spend more time foraging for food. Additionally, evolutionary relationships between species can affect sleep patterns.
