If you have ever stepped off a long-haul flight or worked night shifts, you probably noticed changes in your mood, concentration and general well-being. These changes are the result of disturbances to your natural sleep-wake cycle, your “circadian rhythm”. And disruptions to this cycle can have worse consequences than just feeling a bit moody or distracted. Our latest research shows that it is associated with an increased risk of depression and bipolar disorder.

Circadian rhythms are variations in biological or behavioural processes that recur on a roughly 24-hour cycle. These rhythms occur in body temperature, hormone release and rest-activity patterns, and are thought to be fundamental to our physical and mental health.

Your body has a set rhythm that it works to.
Image: Futurism

Previous studies have investigated the relationship between disruptions to circadian rhythms and mental health, but they have been limited by using only subjective (self-report) measures of sleep and activity, and have typically used very small samples. Also, many of these earlier studies didn’t properly account for other lifestyle factors that might explain poor mental health, such as childhood trauma.

We addressed these issues in our study, published in The Lancet Psychiatry, by using a large sample (over 91,000 participants, drawn from UK Biobank) and by objectively measuring participants’ activity with wrist-worn activity monitors. We also took a wide range of factors into account that might otherwise explain the state of participants’ mental health or cognitive function.

We found that disturbances to circadian rhythms – specifically, increased activity during rest hours and/or inactivity during the day – were associated with a greater risk of mood disorders (such as depression and bipolar disorder), worse subjective feelings of well-being and slower reaction times. These changes couldn’t be explained by age, sex, smoking status, alcohol consumption, average activity levels, education, body mass index and childhood trauma.

Overall, our findings provide strong support for the idea that a disturbed circadian rhythm is associated with poor mental health and well-being. They also suggest that daily activity levels, which can be cheaply and easily measured using a digital activity tracker, could be used to assess people’s risk of developing depression or bipolar disorder.

Growing, global problem

These findings could have important public health implications, as increasing reliance on artificial light, particularly at night, is associated with disturbed circadian rhythms. This is because people exposed to high amounts of artificial light at night are less able to synchronise their rhythms to natural daylight cycles.

It is estimated that almost two-thirds of the world’s population will live in cities by the year 2030. This means that most people will be subject to conditions that can worsen circadian disruption and, by extension, vulnerability to poor mental and physical health.

What our new study can’t tell us is the direction of causality. We don’t know whether circadian disruption leads to poorer mental health and well-being, whether mood disorders lead to circadian disruption, or whether both are linked to other shared genetic or environmental factors.

We hope that future, long-term studies, following up on the same people in the UK Biobank, will be able to address the issue of cause and effect in more detail. It will be important to examine, for example, whether objectively measured circadian disruption can predict new diagnoses or new episodes of mood disorders. It would also be useful to know whether stabilising circadian rhythms in mood disorder patients leads to improved symptoms and well-being.

We have known for a long time that sleep and circadian function are important for physical health, but we are now beginning to recognise their importance for mental health. Part of the work of our research group is to better understand the mechanisms linking circadian disruption and mental ill-health and to use this new knowledge to develop new treatments for conditions such as major depression and bipolar disorder.

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