Sleep is unavoidable. The Guinness World Records notes the longest anyone has stayed awake is 18 days – and it may not even be possible to go beyond that.
At some point you simply can't continue. Animal experiments suggest that you would probably die from sleep deprivation, according to this 2020 study.
It's still not entirely clear why we need to sleep. What we do know is that humans spend about one-third of their lives asleep.
The amount of sleep needed, however, varies greatly from person to person. What counts as normal depends heavily on the individual.
Big differences across age groups
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"There's a wide range," says Ståle Pallesen. He is a psychology professor and a researcher at the University of Bergen's Research Centre for Sleep, Work and Health.
Decades of research have resulted in knowledge about how and how much most people sleep, according to the National Sleep Foundation (NSF) in the USA.
More or less than this is not recommended for adults aged 26 to 64 years.
In other age groups, it's even more extreme. Young toddlers aged one to two may sleep anywhere from 9 to 16 hours – nearly double.
This also means that some people, by nature, really do have more or fewer hours in the day.
But what do researchers really know about these differences? Why is the variation so large?
Families of short sleepers
A whole range of studies point to one key factor, says Ståle Pallesen:
"It's most likely genetics."
You’re mostly born with your sleep pattern, though other influences also matter. More on that later.
In more extreme cases, the genetic role is especially clear.
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There's something called Familial Natural Short Sleep (FNSS).
Some genetic variants passed down in families are linked to a reduced need for sleep. In a large 2019 study, researchers tracked multiple families over generations who carried mutations linked to a reduced need for sleep.
Some only need four hours
People with FNSS typically sleep less than six and a half hours a night throughout their lives and function just fine. Some get by on as little as four hours, according to this 2019 study.
Exactly how this works isn't clear, but FNSS individuals tend to spend less time in both REM and non-REM sleep.
Researchers suspect certain genes influence the brain's waking mechanisms. But they stress that sleep is complex, and sleep mechanisms are unlikely to be controlled by only a few genetic variants.
But these families do indeed have less need for sleep than most others, who are in the range of seven to nine hours per day.
There's also hereditary long sleep. These individuals usually need more sleep than what is considered normal, as described in this study.
Do you feel rested?
The range is so wide because genetics control much of the body, and your body in particular needs a specific amount of sleep.
"We usually say that if you feel reasonably rested and function well during the day, then you're getting enough sleep," says Pallesen.
Research suggests that, for most people, following their natural sleep rhythm is the healthiest path.
Do you feel rested? Or would you have liked a bit more sleep in this inviting bed?(Photo: Shutterstock / NTB)
"Too little sleep in particular is linked to negative health outcomes, such as changes in metabolism, weight gain, and increased mortality," says Pallesen.
Some things can change
Even though genetics play a big role in how long we naturally sleep, it's possible to influence sleep length to some extent.
"If you deliberately set out to cut down on sleep, the body compensates by getting a bit more deep sleep than before," says Pallesen.
He points out that this change can be lasting and has been documented in several studies.
"Environment and behaviour can play a role. It's not 100 per cent genetically fixed, but there's a clear genetic predisposition," he says.
Why do we sleep?
Pallesen explains that sleep has been seen as an evolutionary adaptation to nighttime – dark hours when conditions weren’t favourable for human activity.
If we sleep because of the dark, it might suggest that sleep duration could be adjusted.
"In the past, it was argued that five to six hours was sufficient, and anything beyond that was unnecessary comfort sleep," says Pallesen.
Today, researchers recognise sleep as a vital process for maintaining brain health. The brain undergoes a kind of cleansing during sleep, according to the magazine Science.
"Waste products are flushed out, especially during sleep, from the cerebrospinal fluid in the brain," he says.
This makes sleep essential for human survival. It also suggests that the functions of sleep may differ from person to person, which could explain why our sleep needs are different.
How we handle sleep deprivation varies
Pallesen also points out that sleep is influenced by the environment.
"You can shift people's circadian rhythm. By exposing them to light and darkness at specific times, their rhythm can be advanced or delayed," he says.
But most of the differences are explained by genetics. This includes whether someone is a morning person or a night owl, and how they cope with exhaustion and not getting as much sleep as they need, Pallesen explains.
As a result, people may react very differently to periods of poor sleep – for example, when they have young children.
Some handle it more easily than others, just as some naturally need more sleep than others. In the end, it largely comes down to the cards we're dealt.
