B737-300 FLT CREW OVERSHOT LEVELOFF AS RWY ASSIGNMENT CHANGE WAS BEING PROGRAMMED INTO FMC.

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FO WAS PF STAR INTO DETROIT IN MODERATE TURB TRYING TO MEET XING RESTR OF 12000 FT AND DUE TO THE TURB THE AUTOPLT WAS NOT RESPONDING QUICKLY ENOUGH SO HE DISCONNECTED THE AUTOPLT. JUST AT THAT TIME APCH CTL CHANGED OUR RWY ASSIGNMENT TO RWY 22L. I WAS HEADS DOWN INSTALLING THE APCH IN THE FMC; AS WELL AS SETTING UP THE RADIOS FOR THE APCH. THE FO THEN NOTICED HE HAD INADVERTENTLY DSNDED BELOW 12000 FT TO 11700 FT; AND QUICKLY CLBED BACK UP TO 12000 FT. AT THAT TIME I TOLD ATC THAT WE HAD DSNDED BELOW 12000 FT AND WE WERE CLBING BACK UP TO 12000 FT. AT THAT TIME ATC CLRED US TO DSND TO 11000 FT. WE THEN CONTINUED ON TO THE APCH AND LNDG WITHOUT FURTHER INCIDENT. THE DAY STARTED ON WEST COAST AND HAD OVER-NIGHTED THE PREVIOUS NIGHT ON EAST COAST. I BELIEVE THE TIME ZONE SWITCH COUPLED WITH LONG DAY THAT INCLUDED DELAYS WITH QUICK TURNS; LITTLE TIME FOR FOOD; ADDED TO THE INCIDENT; AS WELL AS THE MODERATE TURB; THE DISCONNECTION OF THE AUTOPLT AT THE TIME OUR RWY WAS SWITCHED. IN THE FUTURE I WILL NOT GO HEADS DOWN UNTIL I'M SURE THE FO COMMUNICATES TO ME THAT HE IS SITUATIONALLY AWARE.