Sometimes doing nothing is the worst action of all, at least when it comes to your health. To see why a sedentary lifestyle is so harmful, scientists asked a group of active, young adults to temporarily stop their daily physical activity, and start living like (sadly) average Americans. The main goal of the study was to see how physical activity affects the way the body regulates the level of glucose—sugar—in the blood. Researchers monitored the volunteers’ blood sugar throughout the day, and tracked their activity and exercise levels. During the first three days of the study, the volunteers followed their normal exercise and walking routines. They exercised at least 30 minutes on most days, and walked 13,000 steps a day—3,000 more than suggested by the American Heart Association. Researchers then asked the volunteers to reduce their activity levels by half, to less than 5,000 steps a day, without changing their diet. Even with just three days of inactivity, the researchers saw significant spikes in blood sugar after the volunteers ate, something that didn’t happen while they were active. “Spikes and swings in blood sugar after meals have been linked to the development of heart disease and Type 2 diabetes,” one of the authors of the study, John P. Thyfault, told The New York Times. The body, however, should regain control of its blood sugar levels once you start moving again. While a few days of inactivity are not harmful, prolonged periods of sitting can set you on a path toward disease. The best advice is to make movement a part of each day.