While poor sleep is often observed as an adverse effect of mental health, a recent study discovered that the correlation between poor sleep and mental illness is more complex.

In a recent publication in Translational Psychiatry, researchers at University College London (UCL) conducted a genetic study and indicated that consistently sleeping less than 5 hours a night may augment an individual’s risk of developing depressive symptoms.

The researchers hypothesized that “there was a significant, unidirectional association between polygenic predisposition to overall sleep duration, short-sleep, and long-sleep duration in the onset of depression during an average 8-year period.”

In a press release on the UCL website, lead author Odessa S. Hamilton, PhD, UCL Institute of Epidemiology and Health Care, stated, “We have this chicken or egg scenario between suboptimal sleep duration and depression, they frequently co-occur, but which comes first is largely unresolved. Using genetic susceptibility to disease, we determined that sleep likely precedes depressive symptoms, rather than the inverse.”

In this study, researchers evaluated genetic and health data from 7,146 individuals recruited by the English Longitudinal Study of Ageing (ELSA), a nationally representative population study in England involving males and females aged 50 years and older. The average age of participants in the study was 65 years.

The researchers evaluated the strength of genetic predisposition among the ELSA participants utilizing findings from previous genome-wide association studies that have acknowledged thousands of genetic variants associated with a greater probability of developing depression and short or long sleep. Additionally, as part of several separate analyses to examine the strength of their results, the research team also explored nongenetic associations between depressive symptoms and sleep duration.

The researchers employed polygenic scores (PGS) and explored the prospective direction involved in suboptimal sleep durations and depression. PGS for sleep duration, short sleep, and long sleep were computed utilizing summary statistics data from the UK Biobank cohort. Sleep duration was categorized as “short-sleep (“≤5h”), optimal-sleep (“>5 to <9h”), and long-sleep (“≥9h”)” and was measured at baseline and across an average 8-year follow-up. At baseline and across an average 8-year follow-up, subclinical depression (Centre for Epidemiological Studies Depression Scale [≥4 of 7]) was also established.

The researchers discovered that one standard deviation increase in PGS for short sleep was linked with 14% greater odds of depression onset (95% CI; 1.03-1.25, P = .008). However, PGS for sleep duration (odds ratio [OR] = .92; 95% CI, 0.84-1.00, P = .053) and long-sleep (OR = .97; 95% CI, 0.89-1.06, P = .544) were not correlated with depression onset during follow-up.

The authors indicated that individuals sleeping 5 hours or less were 2.5 times more likely to show symptoms of depression. Yet, those with depressive symptoms had only a 33.3% increased probability of short sleep. The researchers adjusted for a broad selection of factors that could affect the results, such as education, wealth, smoking status, physical activity, and limiting longstanding illness.

The researchers also discovered a correlation between long sleep and developing depressive symptoms, with participants sleeping longer than 9 hours being 1.5 times more prone to develop depressive symptoms compared with those who sleep an average of 7 hours. However, depressive symptoms were not correlated with sleeping longer 4 to 12 years later, which aligned with the genetic findings.

Overall, the participants in the study had an average of 7 hours of sleep a night, and more than 10% slept for less than 5 hours a night at the initiation of the study period, increasing to over 15% at the end of the study period, and the percentage of participants classified as having depressive symptoms augmented by an estimated 3 percentage points, from 8.75% to 11.47%.

The authors indicated that both sleep duration and depression are partly inherited from one generation to the next, and previous twin studies have proposed that depression is around 35% inheritable and that genetic differences account for 40% of the variance in sleep duration.

Senior author Olesya Ajnakina, PhD, UCL Institute of Epidemiology and Health Care and the Institute of Psychiatry, Psychology and Neuroscience at King’s College London, remarked, “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.”

Andrew Steptoe, PhD, head of behavioral science and health, UCL Institute of Epidemiology and Health Care, stated, “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.”

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