Earlier research had suggested that disrupting these rhythms can adversely affect mental health, but was inconclusive: most data was self-reported, participant groups were small, and potentially data-skewing factors were not ruled out.

For the new study, an international team led by University of Glasgow psychologist Laura Lyall analysed data taken from the UK Biobank – one of the most complete long-term health surveys ever done – on 91,105 people aged 37 to 73.

The volunteers wore accelerometers that measured patterns of rest and activity and had this record compared to their mental history, also taken from the UK Biobank.

Individuals with a history of disrupting their body’s natural rhythm – working night shifts, for example, or suffering repeated jet lag – also tended to have a higher lifetime risk of mood disorders, feelings of unhappiness and cognitive problems, the researchers found.

The results held true even when the potential impact of factors such as old age, unhealthy lifestyle, obesity and childhood trauma were taken into account.

The study, which appeared in medical journal The Lancet Psychiatry, cannot say conclusively that body-clock disturbances are what caused the mental risk, instead of the other way round. But the findings “reinforce the idea that mood disorders are associated with disturbed circadian rhythms,” Lyall said.

Measurements of people’s rest-work cycles could be a useful tool for flagging and treating people at risk of major depression or bipolar disorders, the researchers concluded.

One limitation of the study was the average age of the trial participants: 62.

“Seventy-five per cent of [mental] disorders start before the age of 24 years,” said University of Oxford researcher Aiden Doherty, commenting on the paper.

“The circadian system undergoes developmental changes during adolescence, which is also a common time for the onset of mood disorders,” he added.

Humans have been shown to be either “owls” or “larks”, corresponding to so-called genetic “chronotypes” that determine whether we function better at night or during the day.

Last year, the Nobel Prize for medicine was awarded to three US scientists who pioneered our understanding of how the circadian clock ticks.

Sunburn genes explain why some people burn more

Certain genes can determine which people are more at risk of getting sunburned, and who could possibly develop skin cancer as a result, a study has found.

In a trawl of the genetics of nearly 180,000 people of European ancestry in Britain, Australia, the Netherlands and the United States, researchers found 20 sunburn genes.

Eight of the genes had been associated with skin cancer in previous research, according to findings published in the journal Nature Communications.

And in at least one region of the genome, “we have found evidence to suggest that the gene involved in melanoma risk … acts through increasing susceptibility to sunburns,” co-author Mario Falchi of King’s College London said.

Sun exposure is critical for the body’s production of vitamin D, which keeps bones, teeth and muscles healthy, and which scientists say may help stave off chronic diseases – even cancer. But too much can be painful in the short term, and dangerous for your health.

Hong Kong skin cancer rates set to rise, as victims speak on the risks of a life in the sun

The new study, which claims to be the largest to date into the genetics of sunburn, helps explain why people with the same skin tone can have such different reactions to exposure to sunlight – some burn red while others tan brown. It may also begin to explain factors in skin-cancer risk.

“It is necessary to explore these genes in more detail, to understand the mechanism by which they contribute to propensity to burn,” Falchi said.

In future, the research may help identify people at risk through genetic testing.

“People tend to ‘forget’ that sunburns are quite dangerous,” Falchi said. “Given the rise in incidence in skin cancer, we hope that knowing there is a genetic link between sunburn and skin cancer may help in encouraging people to lead a healthy lifestyle.”

Physical exercise doesn’t halt mental decline in dementia patients

While physical exercise may stave off dementia, it does not delay mental decline in people after they have been diagnosed, a study in nearly 500 people with the condition reported last week.

While a fitness regime improved physical fitness in people with mild to moderate dementia, it “does not slow cognitive impairment”, researchers reported in The BMJ medical journal.

It is generally accepted that exercise can delay the onset of Alzheimer’s and other forms of dementia.

But whether it can slow symptoms after the onset of mental decline has been the subject of much debate.

For the latest study, researchers took 494 people in England who had been diagnosed with dementia, and assigned 329 of them to an exercise programme.

They took part in group sessions of 60 to 90 minutes in a gym twice a week for four months, and home exercises for an additional hour per week. The average age of the group was 77. Participants were assessed at six and 12 months after starting the programme.

The researchers noted that cognition had declined in both the exercise and non-exercise groups.

In the exercise group, the decline was steeper, “however, the average difference was small and clinical relevance was uncertain,” said a press statement.

Commenting on the study, Brendon Stubbs from the Institute of Psychiatry at King’s College London said its findings were “enormously important” for the care of people with Alzheimer’s disease.

“While previous smaller studies have suggested that exercise can prevent or improve cognitive decline in people with Alzheimer’s disease, this robust and very large study provides the most definitive answer we have on the role of exercise in mild-moderate Alzheimer’s disease,” he said.

“The search for effective lifestyle interventions that can delay cognitive decline in dementia must continue.”

How to stave off dementia with these nine lifestyle changes

A separate study among English people aged 65 and older that was published in the journal Jama Psychiatry said people with fewer financial resources appeared to be at higher risk of dementia.

According to the UN’s World Health Organisation, about 50 million people worldwide suffer from dementia, including Alzheimer’s disease – the most common form, making up about two-thirds of cases.

There are about 10 million new dementia cases each year.