Limiting the time we spend sitting to just three hours a day could add an extra two years to our life expectancy, scientists calculate.
Similarly, if we cut daily TV viewing down to two hours we could add on 1.4 years, they say in a report for the online journal BMJ Open.
But experts say the US estimates, which are based on five separate population studies, are too unreliable to predict personal risk.
Plus the targets are unfeasible.
Prof David Spiegelhalter, an expert in risk calculations at the University of Cambridge, said: "This is a study of populations, and does not tell you personally what the effect of getting off the sofa might be.
"It seems plausible that if future generations moved around a bit more, then they might live longer on average.
"But very few of us currently spend less than three hours sitting each day, and so this seems a very optimistic target."
Adults are advised to do at least two-and-a-half hours of moderate-intensity aerobic activity such as cycling or fast walking every week, as well as a couple of sessionsof muscle-strengthening exercises like lifting weights or digging in the garden.
But even if you do this recommended amount, you may still be sedentary - for example, if you work in an office you may spend most of your working day sitting. Muscle-strengthening exercises that work all the major muscle groups should be done on two or more days a week
A growing body of evidence suggests the more time we spend sitting, the less healthy we may be.
Several studies have linked sitting and television viewing to conditions like diabetes and heart disease as well as an increased overall risk of death from any cause.