Although I don't live in Istanbul, Ataturk airport has been a main hub for most of my flights, and therefore I am quite familiar with it.

Turkish Airlines lounge, while very large, it's almost always packed. I was using it at 6am or 6pm, nevertheless it was always jam packed.

Food I don't usually eat there (there is nothing really to eat), I don't even drink much (I don't like the idea of dringking 200ml of water with 10ml of Coca Cola mixed in - that's what you get if you use the Coke machine). The positive side is the relaxation room in which you can take a nap - sometimes. Usually you can't, because it's either full (there are only 4 chairs!), or there would be some inconsiderate travel passengers who would keep pounding on their laptop keyboards in a room where you are supposed to sleep and relax.

So here is my advice to all of you fellow passangers:
In the food court, go upstairs to the lounge level, and on the left you have Millennium lounge. Entrance fee is 36 usd, food is equally crappy, so are the drinks, but what's really good about this lounge are the long bunk beds where you can truly lay down and sleep.

Since I usually have 5-6 hours of waiting, I would first go to the TK lounge check out if there are any free spots in relax room, if not, I would hit the Millennium lounge upstairs and get a few hours of sleep.