AIRSPD DEV FROM ASSIGNED AIRSPD RPTED AS OVER 250 KTS BELOW 10000 FT BY A PIC OF A CRJ INTO ORD; IL.

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WHILE WE WERE DSNDING FROM 7000 FT TO 5000 FT MSL; ATC ASKED WHAT OUR SPD WAS. THE FO REPLIED THAT WE WERE SLOWING THROUGH 250 KTS TO OUR ASSIGNED 170 KTS. I WAS FLYING AND HAD BEEN LOOKING OUTSIDE FOR TFC. WHEN I HEARD THE REPLY; I LOOKED AT THE AIRSPD; WHICH WAS 255 KTS. I ATTRIBUTE MY LACK OF SITUATIONAL AWARENESS TO 2 NIGHTS OF VERY LITTLE SLEEP. THE FIRST NIGHT WAS A REDUCED REST OF 8 HRS BECAUSE WE HAD TO WAIT FOR A CONNECTING FLT ATTENDANT. IN ADDITION TO A LATE DEP; WHEN WE ARRIVED AT OUR DEST; WE WERE SENT TO THE WRONG HOTEL. THIS WAS THE FIRST OVERNIGHT AT A NEW STATION FOR OUR COMPANY. WE MAY HAVE GOTTEN ABOUT 4 HRS SLEEP. THE SECOND NIGHT; WE WERE AT A HOTEL WITH NOISY ADULTS AND CHILDREN RUNNING UP AND DOWN THE HALLWAY UNTIL ABOUT MIDNIGHT. WE MIGHT HAVE RECEIVED 4 HRS SLEEP THAT NIGHT. I COULDN'T RECALL IF WE HAD BEEN GIVEN ANY SPD RESTR PRIOR TO THE 170 KT ASSIGNMENT. AFTER LNDG; I REMOVED MYSELF FROM THE TRIP. I FIGURED THAT IF I WAS FATIGUED ENOUGH TO MISS SOMETHING AS BASIC AS AIRSPD CTL; I DID NOT BELONG ON THE FLT DECK.