Can you keep already cooked food in a crockpot on low?

[JillieSchmillie]

I made a chicken, rice and sauce dish in the crockpot overnight. It's fully cooked now and I've already eaten some. I turned off the crockpot and just left it out while I went to class today. Now I'm home and turned it back on low to heat up some for dinner. I had a bowl and it tasted fine. My question is: can I keep the crockpot on low while I take a nap? Or will it overcook the food? Is it bad to keep reheating it?
It doesn't have a "warm" option...just "off", "high", or "low"