A large study conducted in the UK has linked disruptions to the brain and body’s ingrained daily cycle of rest and activity, known as circadian rhythms, to a greater risk of mental health disorders.

The physiological and genetic mechanisms that tune our awake-sleep phases to match the Earth’s day-to-night phases are so fascinatingly complex that an entire scientific field – called chronobiology – has popped up to study them. But boiled down, our internal clocks are set by an area of the brain called the suprachiasmatic nucleus, which prompts the pineal gland to produce melatonin in response to the amount and quality of light perceived by our eyes.

And although we all live on a 24-hour pattern, a recent wave of chronobiology research has revealed that people are genetically predisposed to experience their peak wakefulness – and corresponding trough of tiredness – at different times of the day, verifying the old-school concept of "morning larks" and "night owls".

Sadly for all us early commuters and late-shift workers, the latest evidence also shows that people trying to function on schedules that don’t match their chronotype become chronically sleep deprived – leading to cognitive impairment and higher risks of anxiety, depression, cancer, diabetes, and cardiovascular disease while we’re alive; culminating in an early death compared with our schedule-blessed counterparts.

Now, while this morose reality is supported by a good number of investigations, most studies followed small numbers of participants and focused primarily on the amount of sleep they got.

The current study, published in the Lancet Psychiatry, analyzed one week’s worth of daytime activity patterns in more than 91,000 volunteers using wearable accelerometers (the tech in Fitbits and Apple watches). Several years later, the same participants filled out questionnaires designed to assess their psychological well-being.

The University of Glasgow-based authors found that subjects whose trackers displayed greater inactivity during the day and/or increased activity at night – a sign that their circadian rhythms could be disrupted – were between 6 and 10 percent more likely to experience major depressive disorder, 3 to 20 percent more prone to bipolar disorder, and around 10 percent more likely to report loneliness and lower levels of happiness, after adjusting for other factors that could contribute to mental health.

"Our findings indicate an association between altered daily circadian rhythms and mood disorders and wellbeing," lead author Dr Laura Lyall stated. "However, these are observational associations and cannot tell us whether mood disorders and reduced wellbeing cause disturbed rest-activity patterns, or whether disturbed circadian rhythmicity makes people vulnerable to mood disorders and poorer well-being."

Other notable limitations of the study are that the data collected does not account for participants’ chronotypes, meaning that it cannot be said definitely whether their activity patterns actually reflect disruptions to their own natural rhythm, and the participants were all older.

The authors conclude that despite its shortcomings, accelerometer data is a cheap and easy way to record rest-activity patterns in large experimental groups. Given that most psychiatric conditions onset in childhood and early adulthood, they hope that future studies focused on young participants can shed light on the causal link between such disorders and circadian rhythms, ultimately allowing for better treatments and prevention.