Televisions don't come with health warnings like cigarettes, but a study suggests that spending hours sitting and watching TV and videos contributes to a shortened lifespan.

Researchers estimate that every hour an adult spends watching TV, his life expectancy shortens by almost 22 minutes. Watching TV for six hours a day can cut short your life by five years.

The study, conducted by the University of Queensland, asserts that watching TV is as bad as smoking or obesity or low physical activity.

Sedentary behavior has been previously linked to an increased risk of dying from cardiovascular disease. However, this study is the first to look how TV habits affect how long you live.

"TV viewing time may have adverse health consequences that rival those of lack of physical activity, obesity and smoking; every single hour of TV viewed may shorten life by as much as 22 minutes.

"With further corroborative evidence, a public health case could be made that adults also need to limit the time spent watching TV," the researchers wrote in their study, published by the British Journal of Sports Medicine.

Dr. Lennert Veerman, lead author of the study, and his colleagues used data from the Australian Diabetes, Obesity and Lifestyle study, which began in 1999 and asked more than 11,000 people aged over 25 about their weekly TV viewing. The researchers then compared the viewing times with Australian mortality rates.

They found that in 2008 Australian adults watched 9.8 billion hours of TV and for every hour spent in front of the box their life expectancy dropped by 22 minutes.

Those who watched TV six hours a day lived 4.8 years less compared to those who watched none.

The researchers say that watching TV is among the most common forms of sedentary behavior. Studies suggest that sedentary behavior is linked to obesity, high levels of blood fats and other heart disease risk factors.

While people should watch TV for the news and for entertainment, too much TV viewing is not good and people need to find alternative light activities, Veerman told AAP.

"They should watch the news and keep themselves informed, but if in the rest of their lives they are pretty active, I wouldn't tell them not to watch a movie," he said.

Last year, another Australian study by Professor David Dunstan and colleagues from the Baker IDI Heart and Diabetes Institute in Melbourne found an hour of TV viewing a day led to an 8 percent increase in the risk of premature death.

While the figures from the study were statistically significant, there was still uncertainty about them because the 11,000 surveyed constituted a small sample, said Veerman.

Veerman said that a recent analysis of all such studies in the Journal of the American Medical Association suggested the risk from TV viewing was lower than their findings in the University of Queensland study. He attributed these differences to different age groups being studied, and a different interpretation of TV viewing. His study only classified someone as watching TV if they were doing nothing else.

Moreover, a direct link between TV viewing and a shortened lifespan is not very probable and the harmful effects of watching TV is almost certainly indirect, said Dr. David L. Katz, director of the Prevention Research Center at Yale University School of Medicine.

"As a rule, the more time we spend watching TV, the more time we spend eating mindlessly in front of the TV, and the less time we spend being physically active. More eating and less physical activity, in turn, mean greater risk for obesity, and the chronic diseases it tends to anticipate, notably diabetes, heart disease and cancer," Katz said, according to a report in Doctors Lounge.

"Physical activity offers huge benefits and these studies back what we already know - that a sedentary lifestyle carries additional risks. We hope these studies will help more people realize that there are many ways to get exercise," England's Chief Medical Officer Sally Davies said.

In the UK, the average amount of time spent watching TV is four hours a day and it is five hours in the United States.