Bananas have the remarkable ability to make you feel better, due to their tryptophan content. This amino acid is transformed into serotonin by the body and is used by the brain to calm down and feel good. Serotonin is often prescribed by doctors to people suffering from depression.
In a study conducted by Simon Young, Young said that "Nonpharmacologic methods of raising brain serotonin may not only improve mood and social functioning of healthy people — a worthwhile objective even without additional considerations — but would also make it possible to test the idea that increases in brain serotonin may help protect against the onset of various mental and physical disorders."
Young, Simon N. "How to Increase Serotonin in the Human Brain without Drugs." J Psychiatry Neurosci 32.6 (2007): 394-99. How to Increase Serotonin in the Human Brain without Drugs. US National Library of Medicine National Institutes of Health, Nov. 2007. Web. 15 June 2012.