Photo: University of Glasgow

Circadian rhythms, the natural variations in our behaviour and activity throughout a 24-hour period, are known to affect everything from hormones to eating habits.

A new study led by the University of Glasgow and published in The Lancet Psychiatry, has found that disrupted circadian rhythms are associated with increased risk of mood disorders, including depression and bipolar disorder. Circadian disruption was also associated with lower subjective wellbeing, higher neuroticism and greater mood instability.

Circadian rhythms are variations in physiology and behaviour that recur every 24-hours, such as the sleep-wake cycle and daily patterns of hormone release. Circadian rhythms occur in plants, animals and throughout biology. They are fundamental for maintaining health in humans, and the integrity of circadian rhythms is particularly important for mental health and wellbeing.

Researchers used activity data on 91,105 participants in the UK Biobank cohort to obtain an objective measure of daily rest-activity rhythms, called "relative amplitude." Individuals with lower relative amplitude were at greater risk of several adverse mental health outcomes, even after adjusting for confounding factors, such as age, sex, lifestyle, education and previous childhood trauma.

“In the largest such study ever conducted, we found a robust association between disruption of circadian rhythms and mood disorders. Previous studies have identified associations between disrupted circadian rhythms and poor mental health, but these were on relatively small samples,” said Laura Lyall, lead author of the research.

In addition to increased risk of depression and bipolar disorder, lower relative amplitude was also associated with low subjective ratings of happiness and health satisfaction, with higher risk of reporting loneliness, and with slower reaction time (an indirect measure of cognitive ability.)

A lower circadian amplitude denotes less distinction, in terms of activity levels, between active and rest periods during the day. This can be due to reduced activity during waking periods or increased activity during resting periods. Shifts in energy levels and sleep disturbances are common during clinical depression and episodes of bipolar disorder.

“This is an important study demonstrating a robust association between disrupted circadian rhythmicity and mood disorders," said professor of psychiatry and senior author, Daniel Smith. "The next step will be to identify the mechanisms by which genetic and environmental causes of circadian disruption interact to increase an individual’s risk of depression and bipolar disorder. This is important globally because more and more people are living in urban environments that are known to increase risk of circadian disruption and, by extension, adverse mental health outcomes.”