iMac and problems with Sleep mode

[MrMacMini]

For the last week it seems that my iMac (24" 2008 model) has been having issues staying in sleep mode.

I put the computer in sleep mode and a few hours later the iMac either wakes up completely or the HDD starts working (screen still off but can hear the HDD working) without anybody physically touching it.

 

[Detrius]

Maybe it's not actually going to sleep to begin with. Run the following to get an idea of whether this is the case:

pmset -g assertions