Eight hours of sleep is probably not necessary for a healthy brain

An overarching analysis of various brain studies shows that shorter sleep probably does not lead to a shrinking brain.

Everyone knows that going to bed too late can make you less sharp the next day. But recent claims go even further, suggesting that regularly getting too little sleep increases the risk of Alzheimer’s disease and even shrinks your brain.

According to one of the most comprehensive analyzes of brain scans to date, that’s probably not true. How much should we really worry about whether we’re getting enough sleep?

Eight hours of sleep

Most health organizations recommend that adults generally need between seven and nine hours of sleep per night. The Dutch Trimbos Institute also uses these figures.

Thanks to the recent availability of wearable wrist-based sleep trackers, it has never been easier to know if you meet this standard. It is therefore not surprising that some people are obsessive about getting enough sleep.

Widely documented animal research, including in NewScientist, shows that the brain’s cleansing system kicks in at night. This rids the body of toxins that contribute to Alzheimer’s disease, such as a protein called beta-amyloid.

Large population studies generally found a link between people with abnormally short or long sleep durations and poorer health outcomes. For example, disturbed sleep behavior has been linked to memory loss and brain shrinkage, one of the hallmarks of Alzheimer’s.

U-shaped curve

Often, the relationship between sleep time and the likelihood of poor health outcomes, such as dementia, follows a U-shaped curve. It is precisely with little or a lot of sleep that the risk of poor health increases. An inverted U-shaped curve occurs in measurements where a higher value indicates better health, such as a larger brain volume.

But such population studies can only find correlations between sleep duration and health. They cannot tell us whether poor sleep is the cause of the health problem.

Only a randomized study could prove that connection, but such a study is almost impossible to conduct. But few people will want to change their sleeping habits for long periods of time for the sake of science, says brain scientist Anders Fjell of the University of Oslo in Norway.

Brain volume and sleep

Fjell and his team delved into a series of studies that used brain volume as a proxy for brain health. First they looked at brain volume in relation to sleep duration at a single moment. They used existing data from approximately 47,000 people. Here they found an inverted U curve, although the highest brain volume was linked to a surprisingly low sleep duration of 6.5 hours per night.

The team then analyzed data from about 4,000 people who had been followed for up to 11 years. In this case, there was no association between sleep duration at the start of the study and brain shrinkage during this period. “It would be very surprising to see this outcome if short sleep actually had a negative effect on the brain,” says Fjell.

Genetic predisposition

The results of the first analysis could also be explained by brain shrinkage causing sleep disruption, rather than the other way around. On the other hand, the difference in brain volume may simply reflect stable differences between people, rather than being the result of brain shrinkage. For example, it could be that people with naturally smaller or larger brains sleep less for unknown reasons, Fjell says.

The researchers also did a third type of analysis. They used the genetic data of approximately 30,000 people, which had been collected in one of the studies. This showed that those who are genetically predisposed to both short and long sleep durations do not have smaller brain volumes than moderate sleepers.

Overall, these results undermine the idea that too little sleep shrinks the brain, Fjell says.

Sleep needs vary

While Fjell doesn’t recommend anyone deliberately change their sleep habits as a result of the findings, he believes there is a lot of natural variation in people’s innate sleep needs. He also thinks that your brain ensures that you get the necessary amount of sleep – unless circumstances do not allow it. ‘We speculate that you are a homeostatic drive get the sleep you need,” he says.

“If you’re tired all day, you’re probably not getting enough sleep, and that can have negative consequences,” says Fjell. “But as long as you feel good during the day, I wouldn’t worry about whether your sleep is six, seven or eight hours long.”

Deep sleep

The team’s conclusions will not convince everyone. Neuroscientist Matthew Walker of the University of California, Berkeley, wrote the book Sleep, in which he advises people to sleep eight hours a night. Walker says the measure of brain health is neuron density, not total brain volume. “Those two measurements are not perfectly correlated with each other, which tells us that they measure different things,” he says.

Walker also says the crucial measure of sleep quality is how long people spend in deep sleep – when brain waves slow down – rather than their total amount of sleep. This is supported by neurologist Maiken Nedergaard at the University of Rochester in New York. Nedergaard discovered the brain system for waste removal, the so-called glymphatic system.

It is during slow wave sleep that the removal of amyloid from the brain becomes more intensive. But most of this takes place during the first four hours of the night, Fjell notes. “It is unclear whether more sleep than that would further facilitate this removal,” he says. ‘From the cleaning systemtheory of sleep does not follow that sleeping for a long time is beneficial.’

Slow wave sleep

Fjell and his team did not look at the duration of slow wave sleep in their study. Collecting such data on this scale would be difficult because the test subjects would have to spend the night in a sleep laboratory with electrodes on their heads.

Nedergaard says this is important work nonetheless, because of the finding that greater brain volume was linked to just 6.5 hours of sleep per night. “The surprise is that the optimal sleep duration is shorter than what is currently recommended,” she says.

Reassuring study

This result therefore suggests that you may not need to worry about sleeping less than eight hours per night. “You shouldn’t be tossing and turning in bed to achieve that goal,” says sleep scientist Michael Chee of the National University of Singapore. Chee says the findings are credible because of the vast amount of data analyzed. ‘This study is reassuring.’