It's this question that may be at the heart of the matter. The kind of happiness you experience - and seek - may matter most to your health. In fact, it may be what defines it. There's a lot we still have to learn about how our heads contribute to bodily health, but here's what we know about the relationship so far.

DEFINING HAPPINESS ISN'T SO EASY

As most people have probably experienced, there are different types of "happy." There's the happiness we get from buying a new iPad and there's the happiness we get from having a fulfilling job that lets us buy the iPad. This fundamental difference is one that researchers have tried to tease apart, and they've described two distinct forms of happiness.

"Hedonic" happiness has to do with pleasure and being satisfied in an immediate sense ("hedonism," of course, comes from the same root). It's about how often you feel good, and experience feelings like excitement, interest, and enthusiasm.

People who are higher in eudaimonic, or long-term, happiness have reduced biomarkers of inflammation, like interleukin-6.

"Eudaimonic" well being, on the other hand, has to do with being satisfied with life in a larger sense; it's about "fulfilling one's potential and having purpose in life," explains Julia Boehm, who studies the relationship between happiness and health at Harvard. How autonomous or self-sustaining you feel, how interested you are in personal growth, the nature of your relationships with other people, whether you have a deep purpose in life, and your degree of self-acceptance are some of the variables that researchers try to measure to get a good idea of whether a person has eudaimonic happiness.

Some scientists have studied the two forms of happiness in the lab and found some significant differences. Work at the University of Wisconsin has found that people who are higher in eudaimonic happiness have reduced biomarkers of inflammation, like interleukin-6. These biomarkers, according to researcher Carol Ryff, are linked to a number of health problems like metabolic syndrome, diabetes, and cardiovascular disease, so lower levels of them might offer a protective benefit. Ryff has also found that having a strong social support network - an integral part of long term life satisfaction - is connected with lower levels of the same biomarkers.

Eudaimonic happiness (or we could call it personal or inner satisfaction) may even help set up the brain differently. Hedonic or short-term happiness has more to do with the kind of "feel good" behaviors that light up the "reward circuits" of the brain, which rely on the neurotransmitter dopamine. These areas are also important in addictive behaviors, which makes sense, since satisfying an addictive craving is the ultimate form of short term (hedonic) pleasure.

On the other hand, when people who are happy in a deeper way (have more long-term, eudaimonic happiness), are faced with negative stimuli, they have more activity in the prefrontal cortex of the brain, which governs executive planning and higher-level thinking. They also have reduced activity in "lower" brain regions, like the amygdala, the seat of our stress and fear responses. People with eudaimonic happiness are also slower to evaluate negative events as being such, which could mean that they are also less likely to "freak out" over real upsets in life.