Antidepressant medications that work on serotonin levels -- SSRIs (selective serotonin reuptake inhibitors) and SNRIs (serotonin and norepinephrine reuptake inhibitors) -- are believed to reduce symptoms of depression , but exactly how they work is not fully understood.

Also, researchers don't know whether the dip in serotonin causes the depression, or the depression causes serotonin levels to drop.

Although it is widely believed that a serotonin deficiency plays a role in depression, there is no way to measure its levels in the living brain. Therefore, there have not been any studies proving that brain levels of this or any neurotransmitter are in short supply when depression or any mental illness develops. Blood levels of serotonin are measurable -- and have been shown to be lower in people who suffer from depression - but researchers don't know if blood levels reflect the brain's level of serotonin.

5. Can diet influence our supply of serotonin?

It can, but in a roundabout way. Unlike calcium-rich foods, which can directly increase your blood levels of this mineral, there are no foods that can directly increase your body's supply of serotonin. That said, there are foods and some nutrients that can increase levels of tryptophan, the amino acid from which serotonin is made.

Protein-rich foods, such as meat or chicken, contain high levels of tryptophans. Tryptophan appears in dairy foods, nuts, and fowl. Ironically, however, levels of both tryptophan and serotonin drop after eating a meal packed with protein. Why? According to nutritionist Elizabeth Somer, when you eat a high-protein meal, you "flood the blood with both tryptophan and its competing amino acids," all fighting for entry into the brain. That means only a small amount of tryptophan gets through -- and serotonin levels don't rise.

But eat a carbohydrate-rich meal, and your body triggers a release of insulin. This, Somer says, causes any amino acids in the blood to be absorbed into the body -- but not the brain. Except for, you guessed it -- tryptophan! It remains in the bloodstream at high levels following a carbohydrate meal, which means it can freely enter the brain and cause serotonin levels to rise, she says.

What can also help: Getting an adequate supply of vitamin B-6, which can influence the rate at which tryptophan is converted to serotonin.