Sitting is as bad for your health as smoking, according to researchers at Belfast's Queen's University.
The researchers found that sitting for long periods of time is linked to increased risk of heart disease, obesity, diabetes and even early death.
Dr Mark Tully, from the UKCRC Centre of Excellence for Public Health at Queen's, said that, on average, people spend over nine hours, or up to 80% of their waking day, sitting down.
Dr Tully also said that levels of sedentary behaviour increase as people age.
The researchers at Queen's are part of a Europe-wide team which is aiming to tackle sedentary behaviour and to increase physical activity in older people.
Working with researchers in Spain, Denmark, Germany, France and Scotland, the four-year study will see researchers develop new ways of helping adults over 65 to sit less and become more active, before testing them on 1,300 people in four European countries.
Speaking on RTÉ’s Today with Keelin Shanley, Dr Tully said 5.3m people worldwide die every year from a lack of physical activity and a sedentary lifestyle, while 5.1 million people die from smoking.
"If you can imagine when you're sitting down you're using very little energy, and you're using very little of your muscles so both your muscles are beginning to detrain and also you're not really using up that excess energy that we tend to eat so it may be accumulating within the body.
"So, it's likely to be leading to diabetes and heart disease through the build-up of plaque in our heart and arteries," he said.
Dr Tully said people can set regular reminders to get up from their desk, or even use a standing desk at work.
"If you think of the number of people that are either sitting in school or university all day, sitting at home all day, sitting in their office all day there's quite a portion of the population that we could help them make small changes that would improve their health," he added.