Via JWZ: 
Every single hour of television watched after the age of 25 reduces the viewer's life expectancy by 21.8 minutes.
By comparison, smoking a single cigarette reduces life expectancy by about 11 minutes, the authors said.
Looking more broadly, they concluded that an adult who spends an average of six hours a day watching TV over the course of a lifetime can expect to live 4.8 years fewer than a person who does not watch TV.
Those results hold true, the authors point out, even for people who exercise regularly. It appears, Dr. Veerman says, that "a person who does a lot of exercise but watches six hours of TV" every night "might have a similar mortality risk as someone who does not exercise and watches no TV." […]
Why a seemingly blameless activity like sitting should be detrimental to health, even for those of us who work out, is not fully understood, although it is assiduously being studied at many labs.
One partial explanation, however, is obvious. "The most striking feature of prolonged sitting is the absence of skeletal muscle contractions, particularly in the very large muscles of the lower limbs," says David W. Dunstan, a professor at the Baker IDI Heart and Diabetes Institute in Australia, senior author of the Australian study, and a pioneer in the study of sedentary behavior.
When muscles don't contract, they require less fuel, and the surplus, in the form of blood sugar, accumulates in the bloodstream, contributing to diabetes risk and other health concerns.