Not just heart disease, but pretty much any way to die. Doing exercise after sitting for 4 hours doesn't help one bit. Some serious implications for online learners but it is actually about as bad if you just spend time writing at a desk or sitting reading a book. It's not the computers per se - it's the fact that people spend too long sitting down in one position and not moving. The recommendation is to get up and move around every 20 minutes but it strikes me that we should probably avoid sitting in one position for a long time as a matter of course. Tablet computers, easily portable notebooks and alternative interfaces (eg Wiimote, voice, etc) seem a potential solution (if we want them to be and make a point of it) as they tend to encourage variety in posture and location.