Jensen explains that he and his colleagues were studying weight control when they discovered that some people "spontaneously start moving round and don't gain weight" when they have overeaten.

It might sound like statement of the obvious, but the killer point is this:

"This really got us thinking about this urge to move," Jensen says, "and how important that might be for maintaining good health."

That led them to a field known as "inactivity research, "which suggests that inactivity, particularly sitting, can be very bad for your health.

Inactivity is bad for you even if you exercise.

When they compared mortality rates of those who spent six hours a day or more sitting and those who reported three or fewer hours and when they took into account other factors such as diet they found something surprising:

Heading to the gym is not a license to spend the rest of the day on your backside.

In 2010, a team led by Alpa Patel of the American Cancer Society in Atlanta analyzed the data from a 14-year study of 123000 middle-aged adults.

It is not clear why there is such a big sex difference.

Extra time on the couch was associated with a 34 percent higher mortality rate for women and 17 percent higher for men in the 14 years after they joined the study.

The message is clear: Sitting still for hour at a time might be a health risk regardless of what you do with the rest of your day. Thank you.

In another study, a team analyzed data on the television viewing habits of 8800 Australians.

They calculated that each hour of television correlated with 22 minutes off the average life expectancy of an adult older than 25.