Disruption to the body clock increases the risk of mood disorders and depression, a large study has confirmed.
Scientists at the University of Glasgow looked at the circadian rhythms - which control functions including sleep patterns, body temperature, our immune systems and the release of hormones - of more than 90,000 people to measure daily rest-activity rhythms, called relative amplitude.
Individuals with lower relative amplitude were found to be 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.
Using mobile phones late at night or waking in the early hours to make a cup of tea were among the bad habits that contribute to "poor sleep hygiene", Daniel Smith, senior author of the paper, told The Times.
"But it's not just what you do at night, it's what you do during the day - trying to be active during the day and inactive in darkness," he said.
"Especially in the winter, making sure you get out in the morning in the fresh air is just as important in getting a good night's sleep as not being on your mobile phone."
Dr Laura Lyall, the study's lead author, said the team had 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."
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.
The study, published in The Lancet Psychiatry, found that 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.
Prof Smith, professor of psychiatry and senior author of the study, 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."
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