How to make the iMac go to sleep after certain amount of time?
The sleep settings have changed due to the new Haswell chips. Because wake-up from sleep is so quick now, and because of the new automatic sleep modes in the CPU, the only thing drawing significant power is the display. Therefore, Apple deemed a sleep setting was only necessary for the display.

Click that last Power Nap checkbox if you want the computer to periodically wake up to do updates and check for mail, etc.