I'd get that carbon monoxide theory checked out pretty smartish. Although it's less likely to be caused by CO entering the cockpit if no-one else seems to be affected, it's also potentially the most dangerous and insidious possibility. It's also probably the easiest possibile cause to confirm/eliminate as cheap CO detectors are widely available.
Mind you, 5 hours for the effects to wear off seems a bit long for low level CO poisoning.
Another thought....you're not flying inverted are you??? Have a look next time. Runway lights 2000ft above you are a dead giveaway.