Also, I avoid using the recirculation button unless the AC isn't capable of blowing cold enough without it or if I'm stuck behind a smelly truck, in an industrial area, etc. Supposedly, recirculating the same air can allow carbon monoxide to build up in the cabin causing headaches and sleepiness. I think you have to have an exhaust leak or a path for exhaust to rise up into the car for it to happen, but I always experienced headaches and sleepiness on long drives with recirc ac in my old Accord, and after I stopped using the recirc those problems went away.