From Scientific American:
Andrew Steptoe and his colleagues at University College London studied the emotional and physical well-being of more than 200 middle-aged Londoners recruited for the Whitehall II psychobiology study in the mid-1980s. The participants underwent stress tests, along with blood pressure and heart rate monitoring, and they were asked to record their feelings of happiness throughout their daily lives. The team found that those people who reported feeling happier more often also had on average lower levels of the stress hormone cortisol, which is linked with hypertension and type II diabetes, than did people who recounted fewer moments of joy. The findings are published online this week by the Proceedings of the National Academy of Sciences.