Mayday - Grave and imminent danger threatening the aircraft or people on board, immediate assistance required.
Pan Pan - Urgent message regarding the safety of the aircraft, someone on board, or of a ship, aircraft or other vehicle or other person within sight.
Based on this definition, pilot incapacitation is most definitely a Pan, and my company ops manual agrees with this. The exception to this I'd say would be if the other pilot was having a heart attack, then someone is in grave danger. If it's just a dodgy curry the night before lock them in the toilet and , erm, Pan!
sorry!