Q Most nights that I leave my iMac to go to sleep, it cannot be woken and seems to have shut down. Is this a known fault?
A Wake from sleep problems commonly result from OS X or firmware bugs, or incompatible third-party extensions.
Ensure that you have brought your iMac fully up to date with firmware and OS X updates. Before the advent of the App Store, it could be quite fiddly to check whether there was an outstanding update, but this is now far easier: the Updates tab of the App Store app should tell you.
You can eliminate problems with most third-party extensions by restarting with the Shift key held, to disable third-party extensions, before leaving it to go to sleep. If that resolves the issue, then you will need to track down what is the exact cause. You will get better clues to that from browsing the logs using Console: it should not be hard to identify the entries prior to your Mac shutting down, and those entries are likely to give more obvious clues.
You should also consider just sleeping your iMac’s display, which will save its hard drive the additional wear and tear of being spun down and spun up again in the morning.
Updated from the original, which was first published in MacUser volume 27 issue 14, 2011.