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 integrity of circadian rhythms is particularly important for mental health and wellbeing.

The 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.

Dr Laura Lyall, lead author, said: “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.”

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 of the day. This can be due to reduced activity during waking periods or increased activity during rest periods. Shifts in energy levels and sleep disturbances are common during clinical depression and episodes of bipolar disorder.

Professor Daniel Smith, Professor of Psychiatry and senior author, said “This is an important study demonstrating a robust association between disrupted circadian rhythmicity and mood disorders.

“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.”

The study, ‘Association of disrupted circadian rhythmicity with mood disorders, subjective wellbeing and cognitive function: a cross-sectional study of 91,105 participants in the UK Biobank cohort’ is published in The Lancet Psychiatry. The work was funded by a Lister Prize Fellowship to Professor Smith.