Those tempted to set up office in the garden as temperatures soar should think twice – or put a hat on, new research suggests.

The study shows sunshine could diminish brain power, making it harder to make the right judgements. 

Working in hot temperatures is already known to cause hyperthermia – a rise in body temperature – which can can cause physical exhaustion. 

But now scientists say prolonged exposure of the head to the sun also affects the brain’s ability to function properly.

A study by university teams in Denmark and Greece said workers could be damaging their brains by working in hot, sunny conditions.

Eight healthy, active males, aged between 27 and 41 took part in the research.

The brain function test consisted of four different computer-based maths and logic tasks that relied on precise calculations. Four heat lamps were positioned to radiate either on the lower-body or on the back, side and top of their heads.

Professor Lars Nybo, of the department of nutrition, exercise and sports at the University of Copenhagen, said: “This study provides evidence that direct exposure to sunlight – especially to the head – impairs motor and cognitive performance.

The decline in motor and cognitive performance was observed at 38.5 degrees, when previous studies had only shown an impact at higher temperatures.

Researchers said those exposed to sunlight for long periods should cover their heads.

Professor Andtreas Flouis from the FAME Laboratory in Greece, said: “It is of great importance that people working or undertaking daily activities outside should protect their head against sunlight.”

Dips in mental performance caused by sunshine could also mean drivers were more likely to make mistakes on the road, he suggested. 

“The ability to maintain concentration and avoid attenuation of motor-cognitive performance is certainly of relevance for work and traffic safety as well as for minimising the risks of making mistakes during other daily tasks,” he said. 

The findings appear in the journal Scientific Reports.