A CPR FLC DSNDING A G1159 NEAR CGT FAILED TO LEVEL AT THE ALT AS ASSIGNED BY ATC.

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WHILE DSNDING FOR LNDG AT MDW; WE WERE PROCEEDING DIRECT TO CGT VOR. LATER WE WERE CLRED TO 'CROSS CGT AT 10000 FT; MAINTAIN 6000 FT.' AS PNF; I SET 6000 FT INTO THE ALT ALERTER FOR A SECOND AND THEN DECIDED TO SET IT TO 10000 FT INSTEAD. I WOULD RESET IT TO 6000 FT WHEN WE PASSED CGT. THE PF WATCHED ME DO THIS. I FELT HE UNDERSTOOD WHY THE ALT ALERTER WAS SET TO 10000 FT AND I CONTINUED MY OTHER DUTIES. A FEW MINS LATER; I WAS SURPRISED TO SEE US DSNDING THROUGH 9500 FT. I PUT MY HAND ON THE YOKE AND SAID 'WE NEED TO MAINTAIN 10000 FT.' WE CLBED TO 10000 FT DIRECT TO CGT AND WERE NOT QUESTIONED BY ATC. THE REASON FOR THIS ALTDEV WAS THE PF SETTING 6000 FT IN THE ALERTER WHILE I WAS INVOLVED WITH OTHER DUTIES. OUR STANDARD PRACTICE OF ALWAYS AGREEING ON THE ASSIGNED ALT BROKE DOWN. HE THOUGHT WE WERE CLRED TO 6000 FT. I MAY HAVE REINFORCED THAT IDEA BY FIRST SETTING 6000 FT; THEN 10000 FT; BUT THE PF FAILED TO SEEK MY CONCURRENCE BEFORE HE RESET IT TO 6000 FT. CONTRIBUTING TO THIS ERROR IS THAT WE FLY THIS RTE OFTEN AND ZAU NORMALLY CLRS US TO 6000 FT BEFORE REACHING CGT VOR. THIS HELPS EXPLAIN WHY THE PF BELIEVED WE WERE CLRED TO 6000 FT. I FAILED TO EMPHASIZE THE 10000 FT RESTR WHILE SETTING IT IN THE ALERTER. I ASSUMED THAT THE PF HAD HEARD ATC CLRLY AND HAD HEARD MY RADIO READBACK. WE HAD BOTH BEEN ON DUTY FOR ABOUT 13 HRS WHEN THIS INCIDENT OCCURRED. I BELIEVE FATIGUE CONTRIBUTED TO THE FALSE ASSUMPTIONS THAT WE BOTH MADE. IT'S CERTAINLY EASIER TO BE COMPLACENT ABOUT COCKPIT PROCS WHEN ONE IS TIRED AND LOOKING FORWARD TO GETTING HOME.