Why aren't people nocturnal

Why some people are night owls

The day only has 24 hours, but for some people the biological rhythm dictates more. Sleep disorders are the result. A common gene variant could be to blame.

They are called larks and owls, people who voluntarily get up early or prefer to be up late at night. Many of them can come to terms with it. But if your biorhythm calls for extremely early or late sleep times, it can be difficult for those affected to adapt to society. Often they suffer from insomnia because they do not fall asleep at night or because they wake up extremely early. One then speaks of a sleep rhythm disorder. Now researchers have found a gene variant that changes the internal clock of its wearer and causes a late sleep rhythm disorder, so one in the style of the owl.1

Jet lag every day

The sleep-wake rhythm is controlled by the internal clock, which follows a roughly 24-hour rhythm and is clocked by sunlight. However, there are indications that the internal clock of early risers runs faster (less than 24 hours) and that of late risers slower (more than 24 hours). Therefore, compared to their fellow human beings, it is their turn sooner or later every day. No matter how hard you try, your internal clock remains shifted like a jetlag.

Now researchers under the direction of Michael Young from Rockefeller University in New York have confirmed this thesis, which so far has mainly been supported by results from animal studies, for the first time in a study with humans. The researchers studied a woman who was suffering from severe insomnia. She fell asleep very irregularly and usually did not fall asleep until the early hours of the morning.

A 24.5 hour day

In fact, her internal clock followed a 24.5 hour period. So that this could be determined, the test person lived for two weeks without external influences, i.e. without sunlight and without a watch, in a sleep laboratory. There she slept and woke freely according to her needs. Meanwhile, the researchers logged their sleep times, measured their body temperature and the hormone melatonin, which is released in the dark and makes you tired.

The researchers then also examined various genes that are involved in controlling the internal clock. They found a noticeable change in a gene called Cry-1. In experiments with skin cells from mice, they showed that this change meant that the molecular biological clocks of the cells also ran half an hour slower.

One could think that half an hour would make little difference, write the researchers in the journal "Cell". However, previous studies have shown that every half hour difference in the period leads to a shift in the sleep phase by 2 to 2.5 hours.

Common gene

To check how much the discovered gene variant (the allele) affects its carrier, the researchers examined other family members of the woman and several unrelated families in Turkey. Since the allele is relatively widespread in Europeans, one in 75 people is a carrier, the researchers quickly found it.

All but one of the 39 affected suffered from insomnia. The exception was a man who worked a lot outdoors and was exposed to a lot of sunlight, which is known to help with this type of sleep disorder.

The results of the study are impressive, says Steven Brown from the University of Zurich, who was not involved in the study. Because the allele is so common and practically all carriers suffer from sleep disorders, it can be assumed that many of these late sleep rhythm disorders are actually due to this variant. Even so, not every night owl carries this allele. On the one hand, there are other genes that can adjust the internal clock; on the other hand, the problem in some people is more related to lifestyle than genetics.