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Though the absolute risk of death was small for everyone, the study showed that "in people who do a similar amount of physical activity, those who sit less will have a lower risk of dying, compared to those who sit more," said van der Ploeg.

It's not clear how marathon sitting sessions can increase the risk of death and illness. Hamilton's research suggests that the loss of muscle contractions that typically occurs while sitting or lying down can suppress production of an enzyme in the skeletal muscle called lipoprotein lipase, or LPL. When lab animals were slightly active, the enzyme was not suppressed, he found.

LPL helps regulate the production of triglycerides, free fatty acids and cholesterol. After a meal, for example, levels of triglycerides and glucose initially rise; then they gradually decline as the body removes and stores the nutrients delivered by circulating blood. "It's theorized that sitting may reduce the efficiency of these processes," said researcher David Dunstan, head of the physical activity laboratory at the Baker IDI Heart & Diabetes Institute in Australia.

"We move from chair to chair throughout the day, from in the home, to the car, at work, to the car and again at home," Dunstan added. "Sitting less may be at least as important as exercising more."

Sedentary behavior may also encourage weight gain - a risk factor for a host of illnesses - by reducing the amount of incidental activity in a person's life. In one recent study by Hamilton, volunteers were asked to sit for a day and fed limited calories. The participants - even those who were lean and exercised regularly - rapidly developed insulin resistance, the key cause of diabetes. The results suggest that "while exercise and healthy diet are good for some things, they do not immunize you from sitting too much," said Hamilton.

Chicago adventurer, running coach and fitness expert Jenny Hadfield routinely exercises, but when she transitioned from managing a corporate fitness center to writing full time, she found she was moving considerably less during the day.

"Whether it's writing or social media, you get so lost in what you're doing, and the next thing you know it's midnight," said Hadfield. "My body was hurting more. I had less mobility in my joints and some weight gain."

Hadfield started getting up every 30 minutes or so to do a small chore. She also placed a board she bought for $15 at Home Depot across the arms of her treadmill. Her computer rests on the board. "It's not a workout, said Hadfield. "It's literally moving one mile an hour and I'm answering email, talking on the phone."

There's no proven solution to the harm of sitting; it isn't clear what time limits would help or the best way to take a break. Learning to use standing and treadmill desks often takes several weeks, and the benefits of walking have to be weighed against the costs and practicality of a treadmill desk or other contraptions, such as an elliptical machine desk that Hammacher Schlemmer sells for $8,000.

Experts say miniature exercise bikes, which allow users to pedal under their desk while sitting in an office chair, do not solve the problem of sitting. Proper posture on stability balls, meanwhile, requires core muscle strength that many people do not have, and sitting on them for long periods of time can exacerbate musculoskeletal conditions, Dunstan said.

"Standing all day is not recommended either and certainly not necessary for better health," van der Ploeg said. "Some tasks are better done standing and some sitting. Finding a nice balance between the two could be good for your health and maybe productivity as well."

Groupon's Hadley, who said he enjoys sitting down when he thinks he has earned it - made his own standing desk nearly two years ago by propping his laptop on top of several black risers he found lying around the office. When he gets tired, he bends one leg at 90 degrees and rests that shin on the desk; then he switches sides. "When I sit down, I tend to relax," said Hadley, who has no plans to return to his chair. "This keeps me more on my game."

Hadley's enthusiasm inspired others in his row. Nima Elyassi-Rad, a senior sales intelligence analyst, also put his MacBook Pro on three risers. He says he stands about 12 hours a day; when he feels fatigued he sits on the arm of his chair. "I'm definitely more alert," he said.

But the two are still in the minority. Taylor Somach, 25, who sits next to Hadley, recently tried standing. "I got tired of it," said Somach, who sits about nine hours a day at his desk and several more at home, watching television. "Besides, I have this fancy Herman Miller chair," he said. "I feel like I should use it."

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STAND-UP ADVICE:

-Change positions every 20 or 30 minutes. Even if you don't want to get a standing desk, simply standing up can help. Sitting increases the pressure on the disks in your back, said Dr. Joel Press, medical director of the Spine and Sports Rehabilitation Center at the Rehabilitation Institute of Chicago. Movement provides nutrition to the disks by helping move fluids in and out, Press said.

-Start small. Before investing in an expensive standing desk, experiment with a low-cost homemade version. Books, monitor risers, shoe risers or even an ironing board can all work. Ronald Thisted, chairman of the Department of Health Studies at the University of Chicago, initially used stacks of academic journals before taking the plunge and mounting his computer on an adjustable sit-stand desk.

-Expect an adjustment period. You'll likely feel tired, and some things - such as typing or using a highlighter while walking on a treadmill - may be more difficult in the beginning. "The first couple weeks were a little uncomfortable, like when you start a new exercise regimen," said Ben Shive, 37, a mobile software developer in Lansdale, Pa. But Shive now has only one complaint: "I need new pants after losing an inch off my waist."

-Wear comfortable shoes. Also try a chef's mat or a standing desk mat to help with foot fatigue.

-Pay attention to your posture. A computer monitor should be at eye level. Your hands, wrists and forearms should be straight and roughly parallel to the floor. The elbows should be bent approximately 90 degrees. "Keep your body weight evenly spaced between both feet and symmetrically between your big toe, little toe and heel," said physical therapist Melissa Kolski, education program manager for the Rehabilitation Institute. "Make sure to keep your pelvis neutral, such that if it was a bucket of water it wouldn't tip forward or backward." If you need a rest break every once in a while, "try propping your foot on a stool or riser," Kolski said.