How to Find Out What's Keeping Your Computer from Going to Sleep [Troubleshooting]

How to Find Out What's Keeping Your Computer from Going to Sleep [Troubleshooting]:
from Lifehacker by Whitson Gordon

So your computer is set to go to sleep after a half hour of inactivity, but the darn thing never actually goes to sleep. Want to figure out what's keeping it awake? Here's a quick command for Windows and OS X that'll let you know. More »