This was driving me mad. Everytime I put my iMac to sleep, like an overexcited child, it refused…

Here’s how to diagnose the cause. Open the terminal & type:

pmset -g assertions

You’ll then get a list looking something like this:

“PreventSystemSleep” is what you’re looking for. In my case the owning process stopping my iMac from sleeping was org.cups.cupsd. Cups is the print system, so after turning off printer sharing in my system preferences pane, my insomniac iMac finally got to sleep.