A review of more than 160 studies has established compelling evidence that happy people tend to live longer and experience better health than their unhappy peers. One study that followed nearly 5,000 university students for more than 40 years found those that were the most pessimistic as students tended to die first. An even longer-term study that tracked 180 Catholic nuns from early adulthood to old age revealed that those that wrote positive autobiographies in their early 20s tended to outlive the nuns that wrote more negative accounts of their young lives.

Source: Applied Psychology: Health and Well-Being.