Consistently sleeping less than five hours a night might raise the risk of developing depressive symptoms, according to a new genetic study by University College London researchers.

Historically, poor sleep has been seen as a side effect of mental ill health, but this study found that the link between sleep and mental illness is more complex.

The study, published in Translational Psychiatry, analyzed data from people with an average age of 65 and found short sleep was associated with the onset of depressive symptoms.

“We have this chicken or egg scenario between suboptimal sleep duration and depression. They frequently cooccur, but which comes first is largely unresolved. Using genetic susceptibility to disease, we determined that sleep likely precedes depressive symptoms, rather than the inverse,” says lead author Odessa S Hamilton, PhD, of University College London Institute of Epidemiology & Health Care, in a release.

For the study, the researchers used genetic and health data from 7,146 people recruited by the English Longitudinal Study of Ageing (ELSA), a nationally representative population study in England.

They found that people with a stronger genetic predisposition to short sleep (less than five hours in a given night) were more likely to develop depressive symptoms over 4-12 years but that people with a greater genetic predisposition to depression did not have an increased likelihood of short sleep.

“Short and long sleep durations, along with depression, are major contributors to public health burden that are highly heritable. Polygenic scores, indices of an individual’s genetic propensity for a trait, are thought to be key in beginning to understand the nature of sleep duration and depressive symptoms,” says senior author Olesya Ajnakina, PhD, of the University London College Institute of Epidemiology & Health Care and the Institute of Psychiatry, Psychology, & Neuroscience at King’s College London, in a release.

The researchers assessed the strength of genetic predisposition among the ELSA participants using findings from previous genome-wide association studies that have identified thousands of genetic variants linked to a higher likelihood of developing depression and short or long sleep.

As part of a number of separate analyses to investigate the robustness of their results, the research team also looked at non-genetic associations between depressive symptoms and sleep duration.

They found that people sleeping five hours or less were 2.5 times more likely to develop depressive symptoms, while people with depressive symptoms were a third more likely to suffer from short sleep. They adjusted for a rich selection of factors that could affect the results such as education, wealth, smoking status, physical activity, and limiting longstanding illness.

The researchers also found a link between sleeping long and developing depressive symptoms, with participants sleeping longer than nine hours being 1.5 times more likely to develop depressive symptoms than those who sleep an average of seven hours. However, depressive symptoms were not associated with sleeping longer four to 12 years later, which corresponded to the genetic findings.

“Suboptimal sleep and depression increase with age, and with the worldwide phenomenon of population aging, there is a growing need to better understand the mechanism connecting depression and a lack of sleep. This study lays important groundwork for future investigations on the intersection of genetics, sleep, and depressive symptoms,” says professor Andrew Steptoe, DSc, head of behavioral science and health at University College London Institute of Epidemiology & Health Care, in a release. 

Overall, the participants in the study had an average of seven hours of sleep a night. More than 10% slept for less than five hours a night at the start of the study period, rising to over 15% at the end of the study period, and the proportion of participants classed as having depressive symptoms increased by ~3 percentage points, from 8.75-11.47%.

Both sleep duration and depression are partly inherited from one generation to the next. Earlier twin studies have suggested depression is about 35% heritable and that genetic differences account for 40% of the variance in sleep duration.

In the study, data on sleep and depressive symptoms were combined from two ELSA surveys conducted two years apart, as sleep duration and depression are known to fluctuate over time.

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