Watching three or more hours of television a day can double your risk of dying young, say researchers.
Scientists studied 13,284 healthy volunteers with an average age of 37 to investigate links between different kinds of sedentary behaviour and premature death.
Over a period of eight years, they recorded 97 deaths from heart disease, cancer and other causes.
The risk of death was twice as high for participants watching three or more hours of TV a day than for those watching one hour or less.
"Television viewing is a major sedentary behaviour and there is an increasing trend toward all types of sedentary behaviours," said lead researcher Professor Miguel Martinez-Gonzalez, from the University of Navarra in Pamplona, Spain.