Whether one is an early bird who hits the sack long before midnight and rises at first light, or a night owl who stays up late and sleeps till midday, is largely a matter of genetics – more specifically of the genes that control our internal body clock. Several of these genes have been identified in animals. Sleep durations is also influenced by genetic factors; studies on twins have shown that as much as 40% of the variation in sleep duration patterns is inheritable. Chronobiologists Professor Till Roenneberg and Dr. Karla V. Allebrandt at LMU Munich, in cooperation with researchers from the Max Planck Institute for Psychiatry and the Helmholtz Center Munich, and colleagues based in Italy and Estonia, have now – for the first time – pinpointed gene variants that have an impact on sleep duration on general populations. In the new study, performed as part of the European research project EUCLOCK, they examined 194 variants from 19 candidate clock genes and analyzed these in relations to sleep duration patterns. They found that variations at the gene CLOCK – have a marked influence on sleep duration. “This finding once again underlines the fact that sleep is a fundamental biological need”, says Professor Roenneberg. “Some people simply require more sleep than others. That does not mean that these individuals are lazy or lack willpower”. (Biological Psychiatry, 9 February 2010)
Although the biological functions of sleep remain a matter of debate, its most important parameters are obviously its quality, time of onset and duration. Sleep quality is strongly affected by external factors such as levels of ambient noise or other sources of stress. Sleep duration and timing, in contrast, are controlled mainly by genetic factors. An internal biological clock regulates a whole series of important biological processes and behaviours on a 24-hour, circadian cycle. In mammals, the central clock is located in the suprachiasmatic nucleus (SCN) of the brain, and is based on the interplay between positive and negative regulators that mediate the periodic synthesis and degradation of certain proteins. The clock is set by information on light levels relayed to the SCN from the retina. Among the traits controlled by the clock are the time of sleep onset and the so-called chronotype, i.e. whether one is an owl or a lark. So far, at least 15 genes have been shown to participate in controlling the circadian clock. However, genes that influence sleep duration had so far eluded detection.
“In our study, which was carried out within the framework of the European research consortium EUCLOCK, we investigated two independent populations – one consistent only of short (< 7 hours sleep) and long (> 8.5 hours sleep) sleepers, and one comprised of all sleep duration cases – totalling ca. 1200 subjects. We have shown for the first time a relation of clock gene frequent genetic variants with sleep duration differences in the general population“, reports Allebrandt. Another particularity of this study is that it did not focus on only one gene, but on 19 genes known to be involved in the biological clock. 194 Single Nucleotide Polymorphisms (SNPs) from those genes were examined. SNPs are sites in the genome at which the DNA sequence commonly shows variation within populations. Whether any such variant is systematically associated with variation in a given behaviour or trait, such as sleep duration, can be assessed using statistical methods.
In this study, a particular variant in a known clock gene was found to be especially prevalent in late sleepers. This is the first time that variation in a clock gene has been robustly associated with sleep duration. The SNPs are located in the gene CLOCK. “CLOCK is known to form part of the circadian clock circuit, the mechanism that makes us larks or owls“, says Roenneberg, who directs the Center for Chronobiology at LMU Munich. “We know from large-scale studies that a particular chronotype is associated with a typical sleep duration, but the new results tell us that CLOCK also has an effect on sleep duration.” The team is now hunting for further genes that have an impact on how long we sleep, in the hope that these will throw further light on sleep as a physiological phenomenon.
Sleep is a physiological necessity, which must be sustained if we are to remain healthy and fit. Chronic sleep deprivation can contribute to the development of serious illnesses, including diabetes, metabolic syndrome and cardiovascular disease. It is now abundantly clear that the need for sleep varies widely between individuals. Long sleepers, who require more than 9 hours of sleep per night, are much more susceptible to the negative effects of sleep deficit than short sleepers, who make do with less than 6 hours sleep. As Roenneberg emphasizes, “long sleepers cannot simply be regarded as being lazy or lacking in willpower”. (ca/suwe)
“CLOCK Gene Variants Associate with Sleep Duration in Two Independent Populations”;
Karla V. Allebrandt, Maris Teder-Laving, Mahmut Akyol, Irene Pichler, Bertram Müller-Myhsok, Peter Pramstaller, Martha Merrow, Thomas Meitinger, Andreas Metspalu, Till Roenneberg;
Biological Psychiatry, 9 February 2010;
Prof. Dr. Till Roenneberg
Center for Chronobiology
Phone: +49 (0) 89 / 2180-75608
Fax: +49 (0) 89 / 2180-75238