My Intel iMac running OS 10.5.6 has suddenly stopped observing the settings in Energy Saver. I have repaired permissions, verified the Mac's hard drive, reset the PRAM,reverted to the default settings and changed them back, all to no avail. I will now have to switch the Mac off at night unless someone ccan supply an answer to this problem!
Have you tried trashing the .plist files? They can become corrupt.
Question is: Which one?
I would start with:
~/library/preferences/com.apple.systempreferences.plist.
(~ = your user folder/name)
Just drag it to the desktop, and restart. If that fixes the problem, then the .plist was corrupted. (You may have to reset all your other preferences though). When you restart, a new plist file will be created.
If it doesn't solve the problem, then drag the old one back to the folder you found it in, and replace the new one - you won't then have to reset your settings.
After the user library preferences, the next place I would try is the universal preferences.
Try: Mac HD/Library/Preferences/SystemConfiguration/com.apple.PowerConfiguration.plist
(NB: NOT in your user folder, but in your HD folder)
Similarly, drag it to the desktop and restart. It will either fix the problem or not. If it does, feel free to trash it. If not, then put it back where you found it.
(You could reverse the above two steps - it probably won't matter which way around you do it. The trick is to find the corrupted file by a process of elimination)
If this doesn't work, then I'm stumped.
Hope this helps.
Michael Corgan 