Conventional wisdom says it's best to "look on the bright side," but does science back up the idea that being optimistic comes with real benefits?

The answer is yes--in moderation. Studies have found that optimism is linked to a healthier heart, a longer lifespan, and better outcomes at school and work, while over-optimism is linked to failure for entrepreneurs starting new businesses.

1. Optimism helps promote heart health.

In a six-month study of 309 patients scheduled for a coronary artery bypass, all patients were given a psychological evaluation that, among other factors, measured their optimism. This study found that optimists were half as likely to need to be hospitalized again after the bypass when compared with pessimists. In a similar six-month study of 298 patients undergoing angioplasty, pessimists were three times as likely to have heart attacks or need repeat angioplasty or bypass procedures.

2. Optimism is linked to survival itself.

A study of 941 senior women and men spanning nine years found that optimists had a 45 percent lower risk of death when compared with pessimists. While it could be argued that poor health understandably leads to pessimism in the elderly, a separate study of 6,959 first-year college students that spanned 40 years found that the most pessimistic had a 42 percent higher rate of death compared with the most optimistic.

3. Optimism is correlated with academic and career success.

Survival is a pretty nice perk, but what about surviving college? During the first year of university, the dropout rate for pessimistic students was about double that of the most optimistic students, according to a study of 480 freshmen.

Furthermore, in a small study of law students, optimism prior to starting law school was linked to higher income 10 years later. Researchers concluded that law school graduates enjoyed an extra $32,667 for each mean item increase in optimism, such as moving from a mean item score of 4 to 5 on the optimism test used for this study.

Potential pitfall: The risk of over-optimism.

In some situations in which failure is very likely, such as starting a business, over-optimism comes at a price. While it's no surprise that entrepreneurs are an optimistic bunch, a study of 386 new entrepreneurs spanning five years found that 33.2 percent of new entrepreneurs overestimated the chances that their new business would succeed.

This is a dangerous matter to be over-optimistic about, as a separate study of 209 participants found a negative relationship between how optimistic a company's founder felt and how successful their business actually was. In that study, the more moderate entrepreneurs were more likely to succeed than the over-optimistic ones.

The takeaway here is clear: Keep it positive, but keep it real.