In an experiment by Dr. Robert Emmons at the University of California–Davis, people who kept a 'gratitude journal,' a weekly record of things they felt grateful for, enjoyed better physical health, were more optimistic, exercised more regularly, and described themselves as happier than a control group who didn't keep journals.

Happy for No Reason: 7 Steps to Being Happy from the Inside Out (ed. Simon and Schuster, 2009) - ISBN: 9781416547730