Those who enjoy life the most are three times more likely to live a little longer than those who enjoy it the least, a study of ageing suggests.
University College London researchers' study of 10,000 English people also suggested future disability and poor health could be predicted by the state of a person's mind.
The team said the effects were "large" and independent of age, sex and wealth.
Happiness could be used to spot people at risk of ill health, they added.
Researchers tracked the psychological well-being of 10,000 people aged 50 to 100 over nine years as part of the university's English Longitudinal Study of Ageing.
They interviewed the participants three times between 2002 and 2011, assessing them using three measures of psychological well-being and testing their enjoyment of life with a series of questions.
They found that those recorded as having the greatest enjoyment of life at first interview were more likely than other participants to still be alive nine or 10 years later.
"The difference between those who enjoyed life the most and those who enjoyed life the least was marked, with nearly three times more people dying in the lower than greater enjoyment group," the study said.
Prof Andrew Steptoe, who led this part of the research, said: "What we have found is over a nine year period that about 20% of people will pass away during this time.
"What we found is that out of those people in the highest third of people with the most enjoyment, 9.9% died. Of people in the lowest third of enjoyment 28.8% of them died.
"This was the case even when factors such as age were taken into account, we still find this protective effect of enjoyment."
Prof Steptoe said this could be happening because the kind of people who are happy are the kind who take care of themselves and are therefore quite healthy.
He also suggested that people of a happy disposition were less stressed.
And he also suggested that environmental factors, such as strong social networks, could be at play.
The report also found that one in six people in England aged over 50 were socially isolated.
The Elsa research team, which comprises researchers from UCL, NatCen Social Research, the University of Manchester, and the Institute for Fiscal Studies, found the wealthier over-50s were half as likely to become socially isolated than the least wealthy.