Stress appears to affect long-term health and biochemistry in some fundamental ways, some of which are connected to the aging process - such as telomere length, chronic inflammation, and immune system function. So what happens when a person is the opposite of stressed? There is reason to believe that being happy over the long term has just as much of a beneficial effect as stress does a negative effect: "A review of more than 160 studies of human and animal subjects has found 'clear and compelling evidence' that - all else being equal - happy people tend to live longer and experience better health than their unhappy peers. ... Its lead author [analyzed] long-term studies of human subjects, experimental human and animal trials, and studies that evaluate the health status of people stressed by natural events. ... We reviewed eight different types of studies, and the general conclusion from each type of study is that your subjective well-being - that is, feeling positive about your life, not stressed out, not depressed - contributes to both longevity and better health among healthy populations. ... A study that followed nearly 5,000 university students for more than 40 years, for example, found that those who were most pessimistic as students tended to die younger than their peers. An even longer-term study that followed 180 Catholic nuns from early adulthood to old age found that those who wrote positive autobiographies in their early 20s tended to outlive those who wrote more negative accounts of their young lives. There were a few exceptions, but most of the long-term studies the researchers reviewed found that anxiety, depression, a lack of enjoyment of daily activities and pessimism all are associated with higher rates of disease and a shorter lifespan."

Link: http://www.sciencedaily.com/releases/2011/03/110301122156.htm