For decades, one of the most prominent public health messages has been that smoking kills. But another everyday habit, far less dramatic and far more socially acceptable, may also be damaging our health: prolonged sitting.

Many people now spend up to ten hours a day seated at desks, in meetings, or in front of screens. It may feel harmless, even unavoidable, but growing evidence suggests that too much sitting is linked to serious health risks, including cardiovascular disease, type 2 diabetes and early death.

People are often told to protect their health by exercising more and eating better. That advice matters, but it misses something important. Even those who meet recommended exercise targets may still face increased health risks if they spend most of the day sitting down.

This is because sedentary behavior and physical inactivity are not the same thing. Physical inactivity means not doing enough moderate or vigorous exercise. Public health guidelines recommend at least 150 minutes of moderate activity a week, such as brisk walking or cycling, or 75 minutes of vigorous activity, such as running. Sedentary behavior, by contrast, refers to long periods of sitting or reclining with very low energy expenditure, whether at a desk, in front of the television, or during a long commute.

. . .