But a new study published yesterday in the Annals of Internal Medicine (which, unfortunately, is available only to Annals subscribers), sheds new light on the issue and sounds a loud, clanging alarm bell about the lasting health risks of prolonged sitting. And, yes, it’s worth sitting and reading this one.

Keith Diaz, an assistant professor of behavioral medicine at Columbia University Medical Center, and colleagues at five other institutions, somehow managed to convince 7,985 people aged 45 and older to wear an Actical accelerometer (made by Philips Respironics)—which measures physical movement and energy expenditure—on their right hips for more than 10 hours a day over a stretch of at least four days. (Most people wore the device for at least six or seven days, Diaz told me in an interview this morning.) Then the team retrieved the devices, crunched the stored data, and determined how often the study subjects actually got off of their butts during that period and for how long—whether they were at home, at work, or someplace else.

Overall, during a typical 16-hour waking day, the four groups spent an average of 12.3 hours being sedentary—with the mean “bout” of uninterrupted butt time being 11.4 minutes.

But then Diaz and crew divided this giant couch-warming cohort into four different quartiles based solely on movement (that is, non-sitting) patterns—and they waited several years to see whether mortality outcomes differed between the groups.

Differ they did.

After a median four years of post-study follow-up, those in the least sedentary quartile (sitting a mean 649 minutes a day in typically 6.5-minute bouts) had a dramatically lower rate of death from all causes than those in the most sedentary group (835 minutes at rest, in periods of relative motionless averaging just under 20 minutes each).

Not surprisingly, those who were more active also tended to be younger, have less body mass, and have fewer health issues (diabetes, hypertension, cardiovascular disease) in general. To account for those differences, the research team did several post-hoc analyses where they controlled for these and other factors (smoking, alcohol consumption, region of residence, education) with three different statistical models. In each case, those who sat the least—and for the shortest periods of duration—had the lowest rate of death from all causes.

Indeed, this duration of couchification is the most telling aspect of the study: Those who got up more frequently—presumably, even to stand and fetch the cable remote…or a glass of water in the kitchen, let us hope—were less at risk. (“Persons with uninterrupted sedentary bouts of 30 minutes or more had the highest risk for death if total sedentary time also exceeded 12.5 hours per day,” observed David Alter, a Toronto researcher who was not involved in the study, in an accompanying editorial.)

So why is prolonged, unbroken sitting so dangerous? Diaz (who uses a standing desk, take note) says he and his colleagues hypothesize that it might interfere with glucose regulation—encouraging a pathological transformation in muscle tissue that may have parallels to diabetes: “The muscles stop working like they’re supposed to and they stop taking up glucose like they’re supposed to,” he says. (That paper is in the current issue of the journal Circulation—and, unfortunately, is also blocked to non-subscribers.)

Whatever the mechanism of action turns out to be, however, the message is clear: Get off your damn butt, and do something.

This essay appears in today’s edition of the Fortune Brainstorm Health Daily. Get it delivered straight to your inbox.