Spending more than an hour and a half in the sun each day could raise your chances of developing depression, dementia and dying early, according to new research.
The findings, published in the journal Public Health, challenge the conventional wisdom sunny weather invariably boosts wellbeing.
Scientists have long understood excessive sun exposure increases skin cancer risk.
However, this study marks the first occasion researchers have established a connection between prolonged sunshine and deteriorating mental health.
The research team advises people to cap their daily sun exposure at 90 minutes to protect both their psychological and physical health.
The study drew on 12 years of health records from 270,000 adults across Britain, with participants having a mean age of 56.
During the monitoring period, nearly 34,000 individuals received diagnoses for at least one mental health condition, while approximately 17,000 deaths were recorded.
Investigators from Huazhong University of Science and Technology in China analysed the data to examine how time spent in sunlight correlated with rates of depression, anxiety, dementia and substance abuse.
The research team advises people to cap their daily sun exposure at 90 minutes
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Although bright light therapy has proven effective for treating certain depressive disorders, no previous research had explored how natural sunlight exposure affects the progression of such conditions over time.
Participants who basked in sunshine for longer than 90 minutes daily faced a 17 per cent elevated risk of depression compared with those who limited their exposure to an hour and a half.
The dementia risk proved even more pronounced, with extended sun exposure associated with a 24 per cent greater likelihood of developing the condition.
Those spending excessive time in the sun also had a 15 per cent higher probability of dying prematurely.
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Another heatwave is due to hit Britain at the end of the month
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When examining multiple diagnoses, the research revealed individuals exceeding the 90-minute threshold were 14 per cent more likely to develop one mental health condition.
They were 20 per cent more likely to be diagnosed with two.
Scientists believe several biological mechanisms may explain the link between prolonged sun exposure and mental health decline.
Seasonal fluctuations in mood-regulating chemicals such as serotonin and dopamine, alongside inflammation and heat affecting the head and neck, are among the potential causes identified by the research team.
The researchers said: "The generation of these chemicals could be influenced by sunlight and their imbalance would lead to the development and progression of mental health conditions."
"The lowest risk was found in individuals with sunlight exposure of 1.5 hours a day.
"From a practical perspective, this optimal duration could serve as a preliminary reference for developing sunlight exposure recommendations for mental health promotion."
This gloomy scientific revelation comes as the UK might be subject to another heatwave, with temperatures threatening to hit 35C.
Experts believe there is another period of very hot weather on its way, similar to the scorching end of May.
Jim Dale, meteorologist for British Weather Services, said: “It is going to turn warmer this weekend, but in the longer term, we are watching temperatures with 32C possible in the next fortnight, or even 34C, 35C.
"This is expected during the second half of the month as high pressure starts to become more dominant.
"The change will be gradual with the next few days turning warm before it potentially turns very warm and hot later in the month."
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