Earlier studies have shown that people with a positive mental attitude have different responses to stress. The new study found that the same people are less likely to suffer from depression, heart disease and diabetes.
Andrew Steptoe, professor of psychology at University College London who led the study, said that happy people rely on a different part of the nervous system that slows the heart rate - and they recover more quickly from stress.
"The evidence suggests that happier people have a reduced risk of various conditions," online edition of Daily Mail quoted him as saying.
Steptoe said: "We found happier people recover more rapidly from stress than less happy individuals. This may be one of the mechanisms through which happier people are less prone to stress-related illness."
Unhappiness is a state characterized by emotions ranging from mild discontentment to deep grief. Studies in the past have shown that unhappiness has adverse impact on health.