ALTDEV ALT OVERSHOT. ACR CREW RECEIVES A DSCNT CLRNC WHILE STILL IN THE CLB AND THE CAPT RESETS THE ALT ALERTER TO THE LOWER ALT CANCELING THE ALT CAPTURE FEATURE OF THE AUTOPLT CAUSING AN OVERSHOOT IN ALT.

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ASSIGNED FL220; PLT'S DISCRETION TO 11000 FT. THE CAPT ASKED ME TO VERIFY SOME INFO WITH THE LEAD FLT ATTENDANT VIA THE INTERPHONE. WHILE I WAS 'HEAD DOWN' ON RADIO PANEL (AND OFF THE ATC RADIO); ATC CLRED US TO DSND. AS I FINISHED MY ASSIGNED TASK AND LOOKED UP; I NOTICED WE WERE CLBING. I QUESTIONED THE CAPT; WHO SAID NO; WE'RE NOT SUPPOSED TO CLB. HE THEN DSNDED. WE GOT ABOUT 400 FT HIGHER THAN ASSIGNED ALT. HE WAS APPARENTLY WORKING ON A HEADSET PROB WHEN THE ACFT STARTED TO CLB. SUPPLEMENTAL INFO FROM ACN 311083: WHILE IN CLB TO FL220; PRIOR TO LEVELOFF; CTR ISSUED CLRNC AT PLT'S DISCRETION TO 11000 FT. THE AUTOPLT WAS ENGAGED AND IN VERT SPD MODE. SETTING 11000 FT DISABLED THE AUTO LEVELOFF AT FL220. I THINK LOW BLOOD SUGAR PLAYED A BIG PART. THE SCHEDULE OF SHORT LEGS AND NO PLACE OR TIME TO EAT PRODUCED A SIT WHERE THE CAPT HAD NOT EATEN IN ALMOST 12 HRS. PLUS THE LONG DUTY DAY PRODUCED FATIGUE. A 1 HR PLUS MECHANICAL DELAY EARLIER DID NOT HELP. NO CREW MEALS ARE CARRIED ON THE ACFT AT THIS AIRLINE. I WAS JUST PLAIN OUT OF IT; AND THE SYMPTOMS WERE INSIDIOUS. THE CTR MADE NO MENTION OF THE ALT BUST; AND THERE WERE NO OTHER ACFT IN THE AREA. FORTUNATELY THE FO HAD EATEN THE ONLY LEFT OVER MEAL EARLIER OR WE MIGHT STILL BE CLBING!