Watching TV for an average of six hours a day could shorten the viewer's life expectancy by almost five years, indicates research published online in the British Journal of Sports Medicine.
The impact rivals that of other well known behavioural risk factors, such as smoking and lack of exercise, the study suggests.
Sedentary behaviour - as distinct from too little exercise - is associated with a higher risk of death, particularly from heart attack or stroke. Watching TV accounts for a substantial amount of sedentary activity, but its impact on life expectancy has not been assessed, say the authors.
They used previously published data on the relationship between TV viewing time and death from analyses of the Australian Diabetes, Obesity and Lifestyle Study (AusDiab), as well as Australian national population and mortality figures for 2008, to construct a lifetime risk framework.
AusDiab is a national survey of a representative sample of the population, starting in 1999-2000, and involving more than 11,000 adults aged 25 or older.
The authors then constructed a risk framework for the Australian population in 2008, based on the answers the survey participants had given, when quizzed about the total amount of time they had spent in the previous week watching TV or videos.
In 2008 the authors estimated that Australian adults aged 25 and older watched 9.8 billion hours of TV, which led them to calculate that every single hour of TV watched after the age of 25 shortened the viewer's life expectancy by just under 22 minutes.
Based on these figures, and expected deaths from all causes, the authors calculated that an individual who spends a lifetime average of six hours a day watching TV can expect to live just under five fewer years than someone who does not watch TV.