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 OS X have an easy method for finding out what the problem is. If you're on Windows, go to Start > Programs > Accessories, right-click on Command Prompt and open it as an administrator. Then type:
It'll let you know if anything is keeping the computer awake. In the screenshot above, for example, it told me that I had a file open in PotPlayer that was preventing my computer from sleeping (since the file resided on the network).
If you're on a Mac, go to /Applications/Utilities, 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. Hit the link below to read more about each method.
What's Keeping My Computer Awake? [Stack Exchange]
Find Out What's Keeping Your Mac Awake [Mac OS X Hints]