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."
It's hard to know what period of time they class as a day? 12 hours, 16 hours?
As the Professor says, less than 3 hours is rather optimistic unless you have a very adtive job where you are on your feet all day AND then do exercise on top!
I have been trying to work out an average for me and it's not easy as I have one job where I'm on my feet moving constantly and then this sitting down one, the hours of which vary most weeks.
I exercise 4/7 days and walk the dog twice a day every day, but then I do like a nice telly slob especailly on a Sunday!
What about you, how long do you think you spend on your butt? I imagine it's easier to guage if you work full time at a desk job.