In puberty, getting up early becomes torture for most teenagers. Scientists from Switzerland wanted to know what is responsible for this: genes or the environment?
Young people are one thing above all else: overtired. Studies show that adolescents get far too little sleep. Instead of at least nine hours, as they actually need, they usually sleep much less. This, in turn, can have consequences for their school performance as well as their health.
The reason for this is a typical shift in their sleep-wake rhythm during puberty. If it were up to puberty, the day would not begin until noon. Nevertheless, the alarm often rings at six a.m. during the week because school starts at eight. The extent to which each individual is delayed and the severity of the sleep deprivation depends on a number of factors. In addition to environmental factors, genetic inheritance also influences the sleep behavior of young people.
Scientists led by Leila Tarokh from the University of Bern in Switzerland wanted to find out which factor has the greatest influence on young people’s sleep during the week, weekends, and holidays.
For their study (1), the researchers monitored 51 adolescents, including 16 monozygotic and 10 dizygotic twins, all around the age of twelve. Over a period of six months, the subjects were monitored using a sleep research method called actigraphy. A special wristband records movement activities and rest phases around the clock.
While sleep behavior hardly differed during weekends and holidays, it looked completely different during the week. On school days, young people slept an average of 8.2 hours, while on days off they slept 8.53 hours. On average, they nodded off 70 minutes later and woke up about 92 minutes later than during the week. Scientists also refer to this shift as social jetlag.
In order to find out whether genes or the environment have the greatest influence on the sleep behavior of young people, the researchers compared the results of the identical twins with those of non-identical twins and with other young people. Identical twins are genetically almost identical. Fraternal twins share about half of their genes. If the results of monozygotic twins are more similar than those of fraternal twins or other adolescents, this would mean that genes play a major role.
In fact, the researchers found a strong genetic influence on sleep length and quality. The researchers calculated that more than 60 percent of the differences between the volunteers were due to the genes – but only on days off. According to the researchers’ theory, on school days the influence of the genes simply cannot unfold through the fixed start of school.
The researchers measured the greatest genetic influence with a factor called the sleep center. This is nothing more than the time after half the sleep duration. Here, 90 percent of the differences on free days are due to genes. The sleep center says a lot about the chronotype, the sleep-wake rhythm of a person. During puberty, young people become more and more like night owls, i.e. late chronotypes that are particularly awake and active in the evening. The researchers’ data suggest that genetic factors are particularly important for this development. Learning more about these genes would probably not only help to better understand the changes during puberty, but they might also provide insight into the development of some mental disorders. Studies suggest that late chronotypes may carry a higher risk of mental disorders such as depression or schizophrenia (2, 3).
This is part 36 of a series covering twin health provided by Paul Enck from the Tübingen University Hospital and science writer Nicole Simon. Further studies in twin research can be found at the TwinHealth website. Translation was done with the assistance of DeepL translator.
- Inderkum AP, Tarokh L. High heritability of adolescent sleep-wake behavior on free, but not school days: a long-term twin study. Sleep 2018;41(3).
- Jones SE, Lane JM, Wood AR, van Hees VT, Tyrrell J, Beaumont RN, et al. Genome-wide association analyses of chronotype in 697,828 individuals provides insights into circadian rhythms. Nat Commun. 2019;10(1):343.
- Antypa N, Verkuil B, Molendijk M, Schoevers R, Penninx BWJH, et al. Associations between chronotypes and psychological vulnerability factors of depression. Chronobiol Int. 2017;34(8):1125-1135.