The study, which is now published in The Lancet Psychiatry, is the first of its kind to use objective measures of activity in a group that is large enough to produce statistically meaningful results.
"Our findings indicate an association," notes first study author Dr. Laura M. Lyall, who's a research associate in the Institute of Health & Wellbeing at the University of Glasgow, U.K., "between altered daily circadian rhythms and mood disorders and well-being."
However, Dr. Lyall also points out that while the findings reveal a strong link, theirs was an observational study, and so they cannot say whether disrupted circadian rhythm increases our susceptibility to mood disorders or whether having mood disorders disrupts our circadian rhythm.
Circadian rhythms and body clocks
Circadian rhythms are the biological and behavioral patterns of living things that follow a roughly 24-hour cycle.
Much of their timing and control lies in the hands of biological clocks, which consist of groups of proteins that reside inside cells.
The genes that tell cells how to make and operate the biological clocks are largely similar in many living species — from fungi to fruit flies and humans.
Changes in the environment are also able to influence an organism's circadian rhythms. A prime example is daylight, which can switch biological clock genes on and off.
There is also a master clock in the brain that keeps all of our biological clocks in sync. It occupies a part of the brain that is directly linked to the eyes.
Our sleep-wake cycle is a major circadian rhythm that responds to light and dark, or day and night. It is also the subject of the new study.
'Relative amplitude' of activity
Disruption of the sleep-wake cycle is a well-known "core feature of mood disorders," as the study authors note, adding that it may also be linked to a higher risk of developing such disorders.
However, previous research has mainly relied on data collected from participants' own reports of their day and night patterns of rest and activity.
It has also tended not to study large groups or take into account sufficient factors that might influence the results.
For their study, Dr. Lyall and her colleagues used data collected by the U.K. Biobank, which is a nationwide research project currently tracking the "health and well-being" of half a million volunteers residing in the U.K.
The data came from 91,105 Biobank subjects aged between 37 and 73 who wore accelerometers for a week during 2013–2015. The devices recorded objective measures of rest and activity 24 hours per day over the 7 days.
From the accelerometer data, the team produced a measure of activity for each person called a "relative amplitude."
A lower relative amplitude is an indicator of disrupted circadian rhythm. For example, someone with reduced activity during the day because of an episode of depression, or increased activity at night because of disrupted sleep, has a lower relative amplitude compared with someone who is active during the day and sleeps soundly at night.
Lower relative amplitude and mood disorders
The scientists then compared the relative amplitude patterns with "mood disorder, well-being, and cognitive variables" that came from mental health questionnaires that had been filled in by the participants.
The findings revealed that participants with lower relative amplitudes of circadian rhythm were the ones most likely to report having a history of bipolar disorder or major depressive disorder.
The team also found reliable links between lower relative amplitudes and:
- more unstable moods
- lower levels of happiness
- higher scores on neuroticism
- greater perceived loneliness
- less satisfaction with health
- "slower reaction times," which they used as a measure of cognitive function
These links were not affected by factors that might influence the results, such as sex, ethnicity, smoking, alcohol, education, body mass index (BMI), childhood trauma, and the time of year in which the activity data were recorded.
The authors recognize that their study was not representative of adolescence, which is typically when most mood disorders begin.
"[M]ore longitudinal studies in younger populations might improve our understanding of causal mechanisms, and help find new ways to predict mood disorders and fine-tune treatments," the authors conclude.
Dr. Aiden Doherty, of the University of Oxford in the U.K., picks up this point in a linked comment article.
As he notes, "Although the U.K. Biobank is one of the most important medical resources worldwide, the study population (median age at baseline of 62 years, IQR [interquartile range] 54–68 years) is not ideal to examine the causes of mental health, given that 75 percent of disorders start before the age of 24 years."
Dr. Doherty suggests, nevertheless, that the Biobank offers a "template" for researching younger populations such as "adolescents and younger adults to help transform our understanding of the causes and consequences, prevention, and treatment of mental health disorders."
"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 well-being and cognitive ability."
Dr. Laura M. Lyall