Watching tv
If an adult watches television for 6 hours per day, he or she can expect to live 4.8 years less than someone who doesn’t watch TV. Every hour of TV that an adult watches after the age of 25 shortens his or her life by almost 22 minutes. In comparison, smoking a single cigarette has been estimated to take about 11 minutes off your life.

Source: British Journal of Sports Medicine, 2012