A happy heart just might be a healthier one as well, new research suggests.
A study of nearly 3,000 healthy British adults, led by Dr Andrew Steptoe of University College London, found that those who reported upbeat moods had lower levels of cortisol, a "stress" hormone that, when chronically elevated, may contribute to high blood pressure, abdominal obesity and dampened immune function, among other problems.
In the study, published in the American Journal of Epidemiology, women who reported more positive emotions had lower blood levels of two proteins that indicate widespread inflammation in the body.
Chronic inflammation is believed to contribute to a range of ills over time, including heart disease and cancer.
Researchers have long noted that happier people tend to be in better health than those who are persistently stressed, hostile or pessimistic. But the reasons are still being studied.
One possibility is that happier people lead more healthful lifestyles, but not all studies have found this to be the case, explained Dr Steptoe.
"We have, therefore, been searching for more direct biological links between positive states and health," he said.
The current findings, according to Dr Steptoe, add to evidence that happiness and other positive emotions are "associated with biological responses that are health-protective". The study, published in the American Journal, included 2,873 healthy men and women between the ages of 50 and 74.
Over the course of one day, participants collected six samples of their saliva so that the researchers could measure their cortisol levels; after taking each sample, participants recorded their current mood - the extent to which they felt "happy, excited or content".
On a separate day, the researchers measured participants' levels of C-reactive protein and interleukin 6, two markers of inflammation in the body.
They found that men and women who reported happier moods had lower average cortisol levels over the course of the day - even when factors such as age, weight, smoking and income were taken into account.
Among women, but not men, positive emotions were also related to lower levels of C-reactive protein and interleukin 6. The reason for the sex difference is not clear, according to the researchers.
Dr Steptoe said the findings on cortisol confirm the results of earlier, smaller studies; the results on C reactive protein and interleukin 6, however, are new.