John Robinson and Steven Martin of the University of Maryland conducted an analysis of U.S. national data of nearly 30,000 adults. They examined the activity patterns of happy and less happy people in the General Social Survey from 1975 to 2006.

The study authors found that happy people were more socially active, attended more religious services, voted more and read more newspapers.

The study, published in the Social Indicators Research, also notes that TV watching requires very little activity compared with other leisure activities. TV viewers don't have to go anywhere, dress up, find company, plan ahead, expend energy, do any work or pay for anything, the researchers said.

The lack of effort required probably accounts for why TV watching takes up more than half of Americans' free time, the study said.

The study found that 51 percent of unhappy people were more likely to have unwanted extra time on their hands compared with very happy people, at 19 percent, and to feel rushed for time -- 35 percent versus 23 percent.