DISRUPTION TO THE body’s internal clock is associated with greater susceptibility to mood disorders such as severe depression and bipolar disorder, the largest study of its kind has found.

The study involving 91,000 people was published in The Lancet Psychiatry and looked at the impacts of disruption to ‘circadian rhythms’, which are physical, mental and behavioural changes in the body that follow a 24-hour cycle.

It found that increased activity during rest periods and/or inactivity during the day are also associated with mood instability, more subjective loneliness, lower happiness and health satisfaction, and worse cognitive function.

The results held true even after adjusting for a wide range of influential factors including age, sex, lifestyle, education, body mass index and childhood trauma.

This study is the first to objectively measure patterns of rest and activity (using accelerometers), and to have sufficient sample size to assess the effect of circadian disruption on various mental health disorders.

“To look at this in more detail, it will be useful for future studies to track participants’ rest-activity patterns over time to see whether disturbed rhythms can predict whether someone is more likely to go on to develop a mood disorder,” commented author Dr Laura Lyall from the University of Glasgow.

Body clock rhythms govern fundamental physiological and behavioural functions – from body temperatures to eating habits – in almost all living beings.

The brain’s internal time-keeping system anticipates environmental changes and adapts itself to the appropriate time of day. Disruption to these rhythms has been shown to profoundly affect human health.

Greater disease risks arising from this disruption have been identified in the brain, pancreas and stress systems.

Previous research has identified associations between body clock disruption and mental health, but these were typically based on self reports of activity and sleeping patterns, had small sample sizes, or adjusted for few potential cofounders.

Researchers analysed activity data in more than 91,000 participants aged 37-73 from the UK Biobank general population cohort to obtain an objective measure of patterns of rest and activity rhythms.

All participants wore accelerometers for seven days between 2013 and 2015 to record their activity. This information was linked to mental health questionnaires to assess symptoms of mental disorders and subjective wellbeing and cognitive function.

Mathematical modelling was used to investigate associations between low relative amplitude (reflecting greater activity during rest periods and/or daytime inactivity) and lifetime risk of mood disorder, as well as wellbeing and cognitive function.

The researchers found that lower relative amplitude was associated with a greater odds of reporting lifetime history of major depression or bipolar disorder. Lower relative amplitude was also found to be reliably associated with greater mood instability, higher neuroticism scores, more subjective loneliness, lower happiness and health satisfaction, and slower reaction time (an indirect measure of cognitive function).

“While our findings can’t tell us about the direction of causality, they reinforce the idea that mood disorders are associated with disturbed circadian rhythms, and we provide evidence that altered rest-activity rhythms are also linked to worse subjective wellbeing and cognitive ability,” said Dr Lyall.