IMac closes down instead of sleeping

Question

My computer frequently tries to shut down when it is sleeping;
I use the power button to send it to sleep and mostly a click on the key board wakes it with the Password window; OK.  Sometimes I will also get a message 'shut down prevented by ( some open application)' or 'disc was not ejected properly'.  and other times by the 'log-in' window.
Becomes a bit frustrating at times waiting for machine to re-start & , as I'm Gettin Old, time is not a thing I want to waste '
I have set my time before sleeping at 15 minutes and shut down at 'never'; I usually close the mac down at the end of my day.
I also have  a MacBook Air running the same OS, with no problems at all.
Would appreciate any suggestions or advice.  Neil  NOBE (Nearly Over Bloody Eighty)

Answer 1

Hi Neil,
Welcome to Apple Support Communities.
I found the following resource that may provide more information:
Mac OS X: Why your Mac might not sleep or stay in sleep mode
http://support.apple.com/kb/HT1776
Some things reset the sleep timers, preventing the computer from sleeping:
- Hard drive access (even if you're not using the computer, a running application might be accessing your drive, such as when downloading a large file or listening to music in iTunes)
- Moving the mouse or using a portable Mac's trackpad, pressing a mouse button, or pressing a key on the keyboard
- Some System Preferences settings (see below)
- Open applications (see below)
- Input devices (see below)
- Expansion cards (see below)
- Additional drives, such as USB, FireWire, DVD, or CD drives (see below)
Thanks,
Matt M.