Sleep Mode Issues

Source: http://lifehacker.com/5924010/how-to-find-out-whats-keeping-your-computer-from-going-to-sleep

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.

A lot of things can keep your computer from going to sleep, like downloading a file, opening a file on the network, or even a disconnected printer with an open job. Luckily, both Windows and Mac OS X have an easy method for finding out what the problem is.

For Windows: Go to Start > Programs > Accessories, right-click on Command Prompt, run as administrator, and type:

powercfg -requests

It'll let you know if anything is keeping the computer awake. 

For Macintosh: Navigate to /Applications/Utilities in Finder, open up the Terminal, and type:

pmset -g assertions

It'll tell you if a device is preventing sleep, and what process that is. Unfortunately, this one seems a little more finicky—some people are having problems where it only tells you that your computer's being kept awake, but it doesn't say by what. If you get stuck in that situation, check out Apple's Help page on the subject. It lists a lot of common possibilities.