2-MAN CREW OVERSHOT ASSIGNED ALT.

Narrative

DEPARTING OFF OF ATW FOR MSP; WE WERE EVENTUALLY CLRED TO 18000 FT. THERE WERE SEVERAL LAYERS AND WE WERE LOOKING FOR AN ALT INBTWN THE CLOUD LAYERS. AT ABOUT 14300 FT WE DECIDED TO REQUEST 14000 FT AS A FINAL BECAUSE 16000 FT WOULD PROBABLY PUT US BACK IN THE CLOUDS. I SLOWED THE CLB RATE TO 350 FPM UNTIL WE HAD A CLRNC TO DSND BACK DOWN TO 14000 FT INSTEAD OF CLB TO 18000 FT. WE GOT OUR CLRNC TO 14000 FT AT ABOUT 14700 FT. I SCROLLED THE AUTOPLT DOWN INTO A DSCNT; BUT SOMEHOW NEITHER ONE OF US CAUGHT THE FACT THE ALT PRESELECT DIDN'T GET CHANGED FROM 18000 FT TO 14000 FT. I WAS DISTRACTED AS I WAS LOOKING AT MY CHARTS THINKING AHEAD TO THE EAU CLAIRE ARR AND NEXT THING I KNOW; ZMP ASKS 'XXX; VERIFY LEVEL AT 14000 FT;' WE WERE THEN AT 13700 FT. WE IMMEDIATELY CLBED BACK UP; BUT WE GOT AS LOW AS 13600 FT BEFORE WE STARTED THE CLB. THE ALT PRESELECT STILL READ 18000 FT. THERE WAS NO TFC IN THE AREA SO THERE WAS NO CONFLICTS. NO ONE SHOWED UP ON TCASII. WE WERE OPERATING ON A 'CONTINUOUS DUTY OVERNIGHT.' WE HAD BEEN ON DUTY SINCE XA40 THE DAY BEFORE. WE HAD A FEW HRS OF SLEEP; BUT NOT A FULL NIGHT'S REST. I THINK THAT THIS MAY HAVE BEEN ONE CONTRIBUTING HUMAN FACTOR.