DC10 CAPT CITES FATIGUE IN SLOW REACTION TO ALT HOLD DISENGAGEMENT AND RESULTANT ALTDEV.

Narrative

THIS FLT WAS THE LAST LEG OF A WK OF FLYING. THE PREVIOUS 2 DAYS SAW THE CREW TRANSITION FROM DAY TO NIGHT FLYING. SLEEP PATTERNS WERE DISRUPTED AS A RESULT. FLT WAS IN LEVEL CRUISE WITH AUTOPLT ENGAGED. WE HAD BEEN LEVEL FOR ABOUT 1 HR. I REACHED INTO MY CHART BAG (BELOW AND TO MY L); THEN; UPON SITTING UPRIGHT AGAIN; I NOTICED A FLASHING AMBER LIGHT THAT INDICATES WE WERE OFF OUR ASSIGNED ALT. THE FLT GUIDANCE NO LONGER INDICATED ALT HOLD BUT; INSTEAD; VERT SPD. BECAUSE I WAS SO SLEEPY; THE EXACT PROB (ABOVE OR BELOW CORRECT ALT) DIDN'T REGISTER. MY EYES SAW THE INSTS BUT THE BRAIN WASN'T INTERPRETING. I MOVED THE VERT SPD CTL AND BROUGHT US BACK TO FL310 AND OBSERVED ALT HOLD ONCE AGAIN. SOME POINTS OF INTEREST ARE: 1) I LOOKED OVER AT THE FO AND NOTED HE HAD FALLEN ASLEEP. 2) I HAVE NO IDEA WHETHER THE AUTOPLT DID THIS ON ITS OWN OR WHETHER THE FO BUMPED THE VERT SPD CTL (MAYBE TO CORRECT OFF ALT BY 50 FT OR SO) AND THEN FELL ASLEEP. I'LL NEVER KNOW HOW THIS HAPPENED. WE GOT UP NEAR FL320 BEFORE THE TREND WAS REVERSED. ATC ASKED US TO 'SAY ALT' APPROX 4 MINS AFTER RETURNING TO FL310. I RESPONDED 'FL310' AND HE REPLIED; 'ROGER.' NOTHING ELSE WAS SAID.