The people who only need four hours' sleep

Some people really are fine with remarkably little sleep. But it can be hard to tell if you're one of those people.

For as long as she can remember, Aimee Pugh Bernard hasn't needed much sleep. Today she tends to go to bed around 22:00 and wake up around 04:30. "I'm wide awake. I cannot stay in bed," she says.

Sometimes Bernard, an immunology professor at the University of Colorado, can't fall asleep right away. Then she'll log only five hours' slumber.

Perhaps most surprisingly, Bernard usually still feels energised the next day. She'll walk her dog, sweat through an hour-long gym session, and clock 1.5 hours of work before 08:00. Friends gawk at Bernard's ability to do so much on so little sleep. But in her family, it's normal – her father and brother are the same.

Most of us, of course, require more. The average adult aged 26-64 years needs about seven to nine hours of sleep in every 24, according to the US National Sleep Foundation's latest review of the evidence.

Why there is such a wide variation in how much sleep people need has been a long-standing mystery. Yet over the last few years, scientists have pinpointed a wealth of ways in which genetics play a role. From the rare mutations carried by people like Bernard, to hundreds of much more common variants, here's why some people get away with very little sleep. In the future, these findings could be used to help protect those who can't get enough from some of the usual consequences. 

In 2019, Bernard learned the likely reason for her own sleep superpower.

A research team from the University of California, San Francisco had discovered a genetic mutation related to sleep duration in humans. Although the people who carried it went to bed at fairly conventional times, they woke up very early, often getting anywhere from four to six-and-a-half hours' sleep a night. After sending a blood sample to the research team, Bernard found out that she was one of them.

This rare, heritable condition is called familial natural short sleep (FNSS). In the years since, several other mutations associated with FNSS have been identified.

One of the biggest mysteries about FNSS is that those who have it seem "fine" on so little sleep, says the team's lead investigator, molecular biologist Ying-Hui Fu. Exactly why is still being worked out.  

There is intriguing evidence that FNSS-linked genetic variations might not only cause short sleep but protect those who have it from the consequences. This includes memory impairment, for example.

One 2019 experiment led by Fu introduced a mutation linked to FNSS in mice. The team found that the mice with a related variant not only slept less, but seemed to be protected from memory deficits. Another test found that mice with some of these variants were less likely to develop a mouse version of Alzheimer's.

In 2014, an international research team investigated a pair of human fraternal twins where one twin had a short-sleep mutation and the other did not. They found that the carrier twin regularly slept about an hour less than their sibling. They also discovered that when both twins were kept continuously awake for 38 hours, the carrier twin had about 40% fewer performance lapses than the other twin.

The researchers also found some intriguing differences between how the two twins slept. Even though the carrier twin slept less, he had almost the same amount of non-REM sleep as his........

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