A happy worker is a healthy worker, say scientists who have uncovered the strongest link yet between a positive state of mind and long term health.

Andrew Steptoe and colleagues at University College London asked more than 200 middle-aged civil servants how many happy moments they experienced during a typical day. The happiest people had lower levels of chemicals which are linked to heart disease and type II diabetes.

Professor Steptoe said: "It has been suspected for the last few years that happier people may be healthier, both mentally and physically, than less happy people. What this study shows is that there are plausible biological pathways linking happiness with health."

The researchers monitored the emotional and physical state of the volunteers at work, at home and in a laboratory. They found the saliva of those who were happiest had lower levels of cortisol, a stress hormone related to conditions including type II diabetes and high blood pressure. Happier men had significantly lower heart rates - down from 76 beats a minute to 68-70. Men and women who were generally unhappy reacted to stress by producing more plasma fibrinogen chemicals in their blood - a strong predictor of cardiovascular disease.

The results are published today in the journal Proceedings of the National Academies of Science.