A bright sunny day has always been thought to put a spring in your step and a smile on your face.

But too much sunshine can lead to depression, dementia and even premature death, a study found.

As beachgoers yesterday soaked up the rays at Lyme Regis, Dorset, amid highs of 23C (73F), researchers warned against exposing yourself to any more than 90 minutes of sunshine a day.

It's well known that too much sun can cause skin cancer, but this is the first time it has been linked to poor mental health and other conditions. 

Seasonal changes in mood chemicals, inflammation and heat exposure to the head and neck are among the possible mechanisms cited by experts.

They said: '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.'

The study is based on 12 years of data on 270,000 Britons and is published in the journal Public Health. 

While bright light therapy can treat depression and other conditions, no previous studies have investigated effects of sunlight exposure on disease progression.

Of the adults, who had an average age of 56, almost 34,000 developed at least one mental illness and there were 17,000 deaths in the period. 

Researchers then compared sunlight exposure with the likelihood of diagnoses of depression, anxiety, dementia, substance abuse and other conditions.

Those exposed to more than 90 minutes a day had a 14 per cent higher risk of being diagnosed with one condition, and a 20 per cent higher likelihood of two conditions.

They also had a 17 per cent higher risk of depression, 24 per cent greater risk of dementia, and a 15 per cent higher chance of premature death, compared to those who had just an hour-and-a-half.

Seasonal variations in mood chemicals like serotonin and dopamine may be to blame.

Researchers from China's Huazhong University of Science and Technology 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.'