AN ACR FLC LEAVES THEIR ASSIGNED ALT WHEN THE PIC COPIES THE WRONG CLRNC N OF ORD; IL.

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ARRIVING IN CHICAGO ON A SHORT FLT FROM DTW; WE WERE AT 8000 FT AND WERE GIVEN VECTORS FOR SPACING ON FINAL FOR RWY 22R. WE HAD RECEIVED MULTIPLE TURNS OF ALMOST 180 DEGS AND WERE GIVEN A HDG OF 170 DEGS. I ALSO UNDERSTOOD A DSCNT TO 7000 FT WHICH WOULD BE NORMAL GIVEN OUR POS. I SET IT IN THE ALT ALERT WINDOW AND POINTED; FOLLOWING SOP. THE FO ACKNOWLEDGED AND DSNDED TO 7000 FT. SHORTLY; ATC GAVE US A TURN AND SAID WE WERE SUPPOSED TO BE A T8000 FT. WE THEN CLBED BACK UP TO 8000 FT. BOTH OF US FELT WE HAD BEEN CLRED TO 7000 FT. THE APCH CONTINUED WITHOUT INCIDENT. THIS WAS OUR 4TH DAY OF FLYING; THE FIRST 2 CONTAINED EARLY MORNING STARTS; AND THIS PARTICULAR DAY WAS SCHEDULED FOR 5 LEGS AND 12 HRS 52 MINS ON DUTY. HAD TO GET UP FAIRLY EARLY TO ARRIVE AT ARPT IN TIME TO RPT. WE ALSO SAT IN DTW FOR 2 HRS 34 MINS BEFORE FLYING THE LAST LEG. I BELIEVE THAT THE LONG DAY COMBINED WITH THE LONG SIT PRIOR TO THE LAST LEG AND IT BEING THE 4TH DAY ON DUTY ALL COMBINED TO LEAVE US NOT AS ALERT AS WE WOULD LIKE TO THINK. I PROBABLY SHOULD HAVE RECONFIRMED THE ALT WITH APCH; BUT DID NOT HAVE DOUBTS ABOUT RECEIVING THE CLRNC. MULTIPLE TURNS ACROSS FINAL ALSO CONTRIBUTED. NEED TO PAY REALLY CLOSE ATTN WHEN OPERATING IN A VERY FAMILIAR AREA.