iMac shuts down completely when its been asleep for a while.

When my iMac has been asleep for a while (while im at work or sleeping), it just turns off instead of staying asleep. I have to turn it back on using the power button instead of waking it up with the mouse or keyboard. I've checked the system preferences display tabs, and can find nothing except the drop down for how long you'd like it to wait until it falls asleep. Is this just something the new iMacs do? My old one never did, it just stayed asleep until i hit the spacebar or wiggled the mouse. If it's something i can stop, how do i go about doing that? I'd like it to just stay asleep instead of shutting down.