The nature of sleep

Every night, we sign away eight hours of our lives to be spent in slumber. Shouldn't we be doing something better with our time?

Illustration by August Hao

Illustration by August Hao

Earlier this year, news of an alarming epidemic made headlines across Australia. This affliction was affecting nearly half of Australians, and had caused 17% of adults to miss work in the past month. In severe cases, it could lead to heart disease, diabetes, depression and even death.

The name of this condition? Sleep deprivation.

According to a recent survey, 33 to 45% of Australians regularly have trouble getting enough sleep. We might struggle to fall asleep, wake up during the night, wake up too early, or start our day feeling tired. Many experts blame modern inventions such as electric lights, smartphones and the internet for disrupting our natural sleep cycles.

We are all familiar with the effects of sleep deprivation. Without enough sleep, we tend to be slower to react, and we make more mistakes. We become more irritable, less empathetic, and have less control over our impulses. A study by the Adelaide Centre for Sleep Research showed that even moderate sleep loss — remaining awake for 17 hours — can have the same effect on hand-eye coordination as having a blood alcohol concentration of over 0.05.

Above all else, though, sleep loss makes us sleepy. We become vulnerable to falling asleep at inconvenient or even dangerous times. Alarmingly, research by the Sleep Health Foundation found that almost 20% of adults have fallen asleep while driving.

So why is sleep loss so detrimental? Why do we need sleep at all? While this might seem like a basic question, it is a topic of ongoing debate among scientists. Sleep researcher Allan Hobson once quipped that the only known function of sleep is to cure sleepiness. Yet, in the same way that eating serves a greater purpose than relieving hunger, sleep must have some other function that justifies its use of our time.

We spend a lot of our time under the sheets, but still don't fully understand why.   Krista Mangulsone/Unsplash  (CC0 1.0)

We spend a lot of our time under the sheets, but still don't fully understand why. Krista Mangulsone/Unsplash (CC0 1.0)


Sleep is a state of decreased responsiveness that, unlike hibernation and states of unconsciousness, can be rapidly reversed. It is regulated by two processes: a homeostatic process and circadian process. The homeostatic process determines how much we need to sleep. The more time we spend awake, the more we need to sleep, and lost sleep is recovered by sleeping longer and more deeply. The circadian process determines how tired we feel based on the time of day.

Sleep is also accompanied by changes in brain activity, which can be recorded as an electroencephalogram (EEG). The EEG can show how deeply someone is sleeping, too — when a person is more difficult to awaken, they show more ‘slow waves’ in their brain activity. Since it is not always practical to poke someone to see how responsive they are, brain activity is considered the best objective measure of sleep.

To gain clues into how and why sleep evolved, scientists have looked beyond humans. All animals seem to sleep, although some have not yet been studied thoroughly enough to be sure. Even nematodes (round worms) sleep, and nematodes last shared an ancestor with humans around 800 million years ago.

One possibility is that sleep evolved as an adaptive state of inactivity. Sleep tends to be associated with reduced movement, not just in humans, but throughout the animal kingdom. Many scientists even include inactivity in their definition of sleep. The purpose of sleep could be to enforce inactivity at times of day when predation risk is high, or when food is difficult to find. While in this inactive state, animals also save energy by lowering their metabolism.

But not all animals lie down and rest each day, the way that humans do. By recording brain activity, scientists have discovered that some animals even sleep on the move. Dolphins and seals can sleep one half of their brain at a time, while swimming. Frigatebirds sleep while flying long distances across the ocean. For these animals, movement is essential; marine mammals must swim to avoid being swept off course, and frigatebirds cannot land on the ocean. The persistence of sleep, even when an animal cannot be inactive, implies that sleep might have some other essential function.

Sleeping is widespread across the animal kingdom.   Alexandru Zdrobău/Unsplash  (CC0 1.0);  Jordan Whitt/Unsplash  (CC0 1.0);  Mr.TinDC/Flickr  (CC BY-ND 2.0)

Sleeping is widespread across the animal kingdom. Alexandru Zdrobău/Unsplash (CC0 1.0); Jordan Whitt/Unsplash (CC0 1.0); Mr.TinDC/Flickr (CC BY-ND 2.0)

The purpose of sleep could, instead, lie within the brain. When we receive information, we form connections between neurons in our brain. These connections, called synapses, allow neurons to communicate more efficiently. However, they also use energy, and too much connectivity can become inefficient and ‘noisy’. But synapses change during sleep, even in simple animals such as nematodes. According to the ‘synaptic homeostasis hypothesis’, sleep gives neurons down-time to prune their synapses, leaving only those that are most important. In other words, we sleep to forget.

These explanations for the function of sleep are not mutually exclusive. Perhaps sleep originally evolved as a state of adaptive inactivity, and other processes later took advantage of this state. There is evidence that sleep has other functions, too. Sleep clears toxic waste from the brain, helps to maintain the immune system, and can be important during early development. So even if sleep originally evolved to protect us from predators, it is still relevant and necessary for humans today.

One aspect of sleep is indisputable: sleep is incredibly diverse. Large hairy armadillos sleep for around 20 hours each day, while goats only sleep for five hours. Great apes build sleeping platforms to lie on, while baboons sleep sitting upright. Some animals are monophasic, sleeping only once per day. Others are polyphasic and have many bouts of sleep. Some birds even have hundreds of ‘microsleeps’ every day, each less than a minute long.

And what about humans? In modern western society, we often try to fit our sleep into as little time as possible. Typically, we have a single ‘episode’ of sleep each night, which lasts six to nine hours. But has it always been this way?

According to historical records, people in pre-industrial Europe had two episodes of sleep rather than one. In his 1840 novel Barnaby Rudge, Charles Dickens wrote: “He knew this, even in the horror with which he started from his first sleep”. A doctor’s manual from 16th century France advised couples to try to conceive “after the first sleep”. These documents, among others, were compiled by historian Roger Ekirch in 2001. Together, they revealed that until the Industrial Revolution people would sleep for a few hours, wake for an hour or two, then return to sleep.

So are humans supposed to sleep in two phases? Intriguingly, when people are exposed to a 14 hour ‘night’ in a sleep laboratory, their sleep becomes biphasic. Research also indicates that light at night can interfere with the circadian timing of sleep. Perhaps our use of electric lights, and our 24-hour lifestyles, have forced our sleep into a single episode.

If so, we would expect people living in communities without electricity to have biphasic sleep. However, this is not always true. Recent studies of traditional communities in Tanzania, Namibia, Bolivia and Argentina found no evidence of biphasic sleep. Some scientists, therefore, argue that biphasic sleep is probably not our ‘natural’, ancestral form of sleep. Instead, humans may have begun sleeping biphasically when they first moved into Western Europe, where the nights are longer.  Others have disagreed with this interpretation, pointing to historical records of biphasic sleep in other, non-western societies.

Sleeping habits differ widely across cultures and individuals.   Vladislav Muslakov/Unsplash  (CC0 1.0)

Sleeping habits differ widely across cultures and individuals. Vladislav Muslakov/Unsplash (CC0 1.0)


Napping is another point of contention. Humans naturally experience a dip in alertness in the mid-afternoon, which could suggest that we are supposed to nap at this time. But even in traditional communities without a full-day work schedule, there is very little evidence of daily napping. People in some traditional hunter-gatherer communities nap around once every two days, while others rarely nap at all.

To many of us, sleep can seem like a waste of time. If you live to 82 years old — the average life expectancy in Australia — you will have spent around 27 years asleep. Those are 27 years that you could have alternatively spent socialising, working, studying, or watching an impressive number of movies.

For this reason, some people have taken napping to the extreme. In the mid 20th century, an inventor and philosopher named Buckminster Fuller experimented with a schedule now known as the Dymaxion Sleep Schedule. In this schedule, he would sleep for only 30 minutes every six hours. More recently, others have tried their own nap-based sleep schedules and shared their experiences online. Some popular schedules include the Uberman schedule (six 20-minute naps per day) and Everyman schedule (3-hour ‘core sleep’ and three 20-minute naps). Both were coined by Puredoxyk, who claims to have been a polyphasic sleeper for most of the past decade.

These schedules have obvious appeal. The Everyman schedule requires only four hours of sleep per day. The Dymaxion and Uberman schedules only require two. The justification for these extreme schedules hinges on two claims: that humans are naturally polyphasic sleepers, and that most of our sleep is usually wasted on “unnecessary” sleep stages. Neither of these claims are supported by sound scientific evidence.

According to the National Sleep Foundation, 18- to 64-year-olds require seven to nine hours of sleep each night. Older adults require slightly less, and teenagers and children require more. Our need for sleep, and its timing, also depends on our genes. Around 1% of people are ‘short sleepers’, meaning they can function perfectly well on only a few hours of sleep each night. Some people who thrive on extreme, nap-based sleep schedules might actually be short sleepers.

There are concerns that our modern Western lifestyles are causing us to sleep less than we should. Staring at bright screens in the evening can shift the timing of our internal biological clock, causing us to feel more awake and affecting our subsequent sleep. Modern technology can also affect sleep simply by motivating us to stay awake. Prior to electric light, activities such as team sports were restricted to daylight hours. Today, stadiums across Australia are floodlit every night. We now live in a 24-hour society, with nearly 25% of Australians reporting that their daily routine often prevents them from getting enough sleep. 

Our modern lifestyles, including the proliferation of technology, may be disrupting our regular sleep cycles.   Dennis Skley/Flickr  (CC BY-ND 2.0)

Our modern lifestyles, including the proliferation of technology, may be disrupting our regular sleep cycles. Dennis Skley/Flickr (CC BY-ND 2.0)


These concerns are not new; similar complaints were apparently made as long ago as the 1880s. But not all of these claims are well-founded. People in traditional communities, whose lifestyles resemble those of our early African ancestors, sleep even less than people in Western populations. Sleep in these communities is also surprisingly inefficient; the Hadza people of Tanzania spend around nine hours in bed each night, but sleep for only about six.

Yet the people in these traditional communities are remarkably healthy; very few report having sleep problems. The San of Namibia and the Tsimane of Bolivia don’t even have a word for ‘insomnia’ in their native language. 35 out of the 37 of Hadza people who participated in sleep research said that they got “just enough sleep”, as opposed to “too much” or “not enough”. Interestingly, some experts believe that anxiety about sleep is contributing to sleep problems in the Western world. Research has even shown that believing that you slept well — even if you didn’t — can improve your performance the next day.

This is not to say that our modern lifestyles are harmless. There is plenty of evidence that bright light in the evening can delay and disrupt our circadian rhythms. In the Hadza people, a good night’s sleep might also be aided by bright morning light, consistent waking times and cool bedrooms. Still, the variation we see in human sleep — throughout history, and through cross-cultural comparisons — demonstrates flexibility in our sleep that we rarely acknowledge.

Sleep is not a waste of time, and we should not treat it as such. But expecting ourselves to fall asleep instantly, sleep for eight hours straight, and then wake up feeling refreshed, might be unrealistic and even counterproductive. If we can appreciate and allow time for sleep, there is no cause for alarm. Even sleep is not worth losing sleep over.

Edited by Andrew Katsis and Sara Paradowski