When my Imac goes to sleep it won't wakeup. I have to manually torn off power and restart

First what model iMac are you using and what version of Mac OS10 is installed.
There is a chance that this issue has been addressed with a software update or a firmware patch, run your software updates until you have applied all available updates, this should help if your problem has been addressed by Apple in a OS update or firmware patch. Also it is a good idea to reboot from time to time, personally I reboot my MacBook Pro every time it comes out the bag, and desktops every two days. Resetting P-RAM (http://support.apple.com/kb/HT1379?viewlocale=en_US) and resetting the SMU or PMU (http://support.apple.com/kb/ht1712) (different nomenclatures for the same thing the power management unit for the Imac) may help. Not knowing you hardware or OS I can not get into more detail however here are (https://discussions.apple.com/thread/2223188?start=0&tstart=0) some links to threads in the apple support forum that may help