Sitting down for more than three hours a day can shave a person's life expectancy by two years, even if he or she is physically active and refrains from dangerous habits like smoking, according to a study to be published on Tuesday in the online journal BMJ Open.

Watching TV for more than two hours a day can exacerbate that problem, decreasing life expectancy by another 1.4 years, said the report, which analyzed five underlying studies of nearly 167,000 people over a range of four to 14 years.

The...