Playing videogames for too long without a break can kill you. In fact, one 20-year-old man in England died recently after spending all night playing games on his Xbox 360.

What killed him? Deep-vein thrombosis – or DVT. Which is what happens when a blood clot forms - usually in the legs – and can break loose, causing a stroke, heart attack or pulmonary embolism. It can happen from a night of gaming, on long plane flights, or if you sit all day at your computer working or surfing the Internet. In fact, DVT nearly killed tennis star Serena Williams after a long plane flight, and kept her off the tennis tour for nearly a year.

What’s the fix? No matter what you’re doing, if you’re sitting, take a break at least every hour.  And stand up, walk around, or hit the bathroom. And drink a big glass of water before you sit down again. That’s because dehydration thickens your blood, and raises your risk for DVT.