(CNN) - Happiness. You know it when you see it, but it's hard to define.
You might call it a sense of well-being, of optimism or of meaningfulness in life, although those could also be treated as separate entities. But whatever happiness is, we know that we want it, and that is just somehow good.
We also know that we don't always have control over our happiness. Research suggests that genetics may play a big role in our normal level of subjective well-being, so some of us may start out at a disadvantage. On top of that, between unexpected tragedies and daily habitual stress, environmental factors can bring down mood and dry up our thirst for living.
Being able to manage the emotional ups and downs is important for both body and mind, said Laura Kubzansky, professor of social and behavioral sciences at Harvard School of Public Health.
Why be happy?
Many scientific studies, including some by Kubzansky, have found a connection between psychological and physical well-being.
A 2012 review of more than 200 studies found a connection between positive psychological attributes, such as happiness, optimism and life satisfaction, and a lowered risk of cardiovascular disease. Kubzansky and other Harvard School of Public Health researchers published these findings in the journal Psychological Bulletin.
It's not as simple as "you must be happy to prevent heart attacks," of course. If you have a good sense of well-being, it's easier to maintain good habits: Exercising, eating a balanced diet and getting enough sleep, researchers said. People who have an optimistic mindset may be more likely to engage in healthy behaviors because they perceive them as helpful in achieving their goals, Kubzansky said.
Lower blood pressure, normal body weight and healthier blood fat profiles were also associated with a better sense of well-being in this study.
For now these studies can only show associations; they do not provide hard evidence of cause and effect. But some researchers speculate that positive mental states do have a direct effect on the body, perhaps by reducing damaging physical processes. For instance, another of Kubzansky's studies found that optimism is associated with lower levels of inflammation.
If what you mean by happiness is specifically "enjoyment of life," there's newer evidence to support that, too. A recent study in the Canadian Medical Association Journal found that people ages 60 and older who said they enjoyed life less were more likely to develop disability over an eight-year period. Mobility was also related to enjoyment of life. This study does not prove that physical problems are caused by less enjoyment of life, but suggests a relationship.