iMac "deep sleep" problem

Hi, I have a iMac 27" Late 2012 and it keeps going in "deep sleep" when I leave it unattended during the day. When I come back, I have to wait for my mac to load the saved state which is quite long. I had many macs in my life and it's the first time I encounter the problem. I've seen that sleep mode before when my MacBook would be out of battery, but never on a desktop... My energy saver settings are set to never sleep (computer) and 15 minutes (display). My setting in Privacy/Advanced/Log out after XX min of inactivity is also off. I checked my hibernation mode in the terminal and it is set to 0. Out of ideas. Anyone ?

Thank you !