People really do get hot and bothered in warm weather, new research has shown.
A study has found stress hormones rise in tandem with the thermometer.
The discovery sheds fresh light on a phenomenon that has puzzled scientists for years.
Dubbed the ‘summertime blues’ there is a wealth of evidence from the past few decades linking heat exposure to aggression, suicide and violence.
Now a Polish team has found this all boils down to the fact that levels of the stress hormone cortisol are lower in winter than summer, and the heat-driven rise makes us tetchy.
A team at Poznan University of Medical Sciences in Poland has found levels of the stress hormone cortisol are lower in winter than summer, and as levels rise with heat, we get more tetchy
It could have implications for public health because the chemical is vital to regulating sugar, salt and fluids throughout the body.
Dr Dominika Kanikowska, a pathophysiologist at Poznan University of Medical Sciences, said having more circulating when it is warm was a surprise.
She said: ‘These non-intuitive findings contradict traditional concepts of the taxing physical toll of winter and the relaxed ease of summer.’
The original data that first associated heat with hostility came from crime statistics. Analyses noted those involving violence increased in summer – especially when it was warmer than average.
Numerous theories have been suggested including raised temperatures causing an increase in heart rate, testosterone and other metabolic reactions that trigger the sympathetic nervous system.
This is responsible for the fight-or-flight response – so people are more inclined to fight.
Dr Kanikowska and colleagues say the reason is simple: it all boils down to the effect of the weather on cortisol.
It is referred to as the ‘stress hormone’ because it is released into the bloodstream during difficult or upsetting situations.
Dr Kanikowska said: ‘The hormone helps reduce inflammation and is essential for maintaining overall health.
‘Cortisol levels are typically highest in the morning and gradually drop throughout the day. Levels are lower in the evening to maintain healthy sleeping patterns.
‘Illness, lack of sleep and certain medications can affect cortisol levels more than normal daily fluctuations.’
Her researchers studied a group of female medical students on two separate days in both winter and summer. They found cortisol levels to be higher on the latter dates. Inflammation levels did not change significantly between seasons.
Dr Kanikowska said there have been studies into the seasonal variability of the hormone but these have shown inconsistent results.
This is possibly because participants were tested in their own homes and not in a uniform setting.
She said her study presented at an American Physiological Society meeting in San Diego was the most thorough of its kind.
They took saliva samples every two hours during each testing period – a full 24-hour cycle – to measure levels of cortisol and markers of inflammation.
The volunteers also completed a lifestyle questionnaire during each session about their sleep schedule, diet and physical activity levels.