Some people really only need four hours of sleep. What genetics says about it
Most adults need seven to nine hours of sleep. But there are people who get enough sleep in just four to six hours and do not feel tired. Scientists have found genetic characteristics that may explain this ability, writes BBC Future.
Rare short-sleep genes
In 2009, researchers at the University of California, San Francisco, discovered one of the rare hereditary mutations associated with this trait. It is called familial natural short sleep (FNSS). Carriers of the relevant mutations usually go to bed at their usual time but wake up much earlier than others and do not feel the need to sleep further.
Since then, scientists have identified several more related genetic mutations. However, the main mystery remains: why can these people sleep so little and yet not experience negative consequences?
Studies show that some mutations may not only reduce the need for sleep but also protect the body from its deficiency. For example, in a 2019 experiment, scientists introduced one such mutation into mice.
The animals started sleeping less but became more resistant to memory impairments. Other experiments have shown that some similar genetic variants can also protect mice from changes characteristic of Alzheimer's disease.
Interesting results were also obtained in a study of two twin brothers, only one of whom had a short-sleep-related mutation. He regularly slept about an hour less than his brother. Scientists conducted an experiment during which both brothers stayed awake for 38 consecutive hours, and their attention and performance were periodically tested. It turned out that the mutation carrier made about 40% fewer errors than his brother.
Moreover, although one of the brothers slept less, the duration of slow-wave sleep, including deep sleep, was almost the same for both. It is possible that the body of the mutation carrier is able to use the time allocated for sleep more efficiently.
For now, this is just a hypothesis. Scientists still need to understand exactly what processes allow some people to rest fully in a shorter amount of time.
Not all short-sleep genes are equally beneficial
Rare mutations that allow only a few hours of sleep make up only a part of the complex genetic picture. In fact, hundreds of genomic regions affect sleep duration. According to various estimates, heredity can explain 30 to 65% of differences in sleep duration between people.
In a new study, geneticists analyzed data from nearly 650,000 adults and identified about 244 genomic regions associated with sleep duration. Each of the common genetic variants individually affects it insignificantly, on average by about a minute. But their combined effect can explain a difference in sleep duration between people of about 20 minutes. At the same time, a genetic predisposition to sleep less does not necessarily mean that the body tolerates sleep deprivation better.
For example, scientists studied 343 women with bipolar disorder who had children. Those with the highest genetic predisposition to short sleep had half the risk of postpartum psychosis compared to women with the lowest predisposition. Perhaps their bodies are better adapted to sleep disturbances after childbirth.
But studies on broader population groups have also found the opposite pattern. Some genetic variants associated with shorter sleep are simultaneously associated with an increased risk of insomnia, fatigue, depressive symptoms, and type 2 diabetes.
We may not notice sleep deprivation
Distinguishing a natural need for short sleep from ordinary sleep deprivation is harder than it seems. The main problem is that a person is not always able to objectively assess their condition.
In one experiment, healthy adults were divided into groups. For two weeks, one group was allowed to spend only four hours in bed per night, the other six. With each day, the results of tests for attention and mental performance worsened. However, the participants themselves rated their sleepiness as minor, even when the accumulated impairments were already serious.
Thus, feeling refreshed after short sleep does not yet prove that the body has truly had enough rest. Scientists hope that further study of genetics will help distinguish between people who naturally need less sleep and those who are simply accustomed to sleep deprivation. In the long term, this could lead to the creation of drugs that help the body better tolerate sleep deficiency, but for now, the mechanisms of such protection are insufficiently studied.