Article: Why Luck Matters More Than You Think.
"... they asked a first group of people to keep diaries in which they noted things that had made them feel grateful, a second group to note things that had made them feel irritated, and a third group to simply record events. After 10 weeks, the researchers reported dramatic changes in those who had noted their feelings of gratitude. The newly grateful had less frequent and less severe aches and pains and improved sleep quality. They reported greater happiness and alertness. They described themselves as more outgoing and compassionate, and less likely to feel lonely and isolated. No similar changes were observed in the second or third groups. Other psychologists have documented additional benefits of gratitude, such as reduced anxiety and diminished aggressive impulses."