Happiness is good for the heart. It thins the blood, reducing its stickiness, and cuts the level of the stress hormone cortisol, according to psychologists from University College London. The more moments of happiness people experience the better their health is likely to be, they say.

While the adverse effects of depression and anxiety on the body are well known, the biological impact of a good mood has not been demonstrated before, they claim. The finding, published online in Proceedings of the National Academy of Sciences, comes as separate research shows that the risk of heart disease in adults is increased by infections such as colds and flu suffered in childhood.

Researchers found the flexibility of the arteries in 600 children studied was reduced during an infection and in some cases did not recover after the infection was over. Artery flexibility is a key risk factor for heart disease and the researchers suggest in the journal Circulation that there could be a link between this and infections from childhood.

In the study examining the impact of happiness on health, 116 middle-aged men and 100 women from London were monitored at work and leisure and tested in a laboratory. Blood and saliva samples were taken and they were asked to rate their happiness at different points during the day.

Some of the participants never felt happy while others felt happy all the time. Most were happiest during their leisure hours. The main difference found in the study was lower levels in the happier people of fibrinogen, a clotting factor in the blood which increases the risk of heart attack.

Andrew Steptoe, professor of psychology, who led the study, said: "What we find particularly interesting is that the associations between happiness and biological responses were independent of psychological distress.

"We already know that depression and anxiety are related to increased physical health risk. This study raises the intriguing possibility that the effect of happiness may be somewhat separate."