CL65 FLT CREW MISSES CROSSING RESTRICTION ON DESCENT INTO ORD CITING FATIGUE AS THE PROBLEM.

Narrative

WE WERE AT 16000 FT ON THE KNOX ARR INTO ORD AND WERE GIVEN THE XING RESTR FOR BEARS INTXN AT 11000 FT. I VERIFIED THAT THE CAPT HAD PROPERLY ENTERED 11000 FT INTO THE ALT PRE-SELECT AND NOTED THAT HE HAD NOT IMMEDIATELY BEGUN THE DSCNT; BUT WE WERE ALSO NOT TO THE POINT WHERE IT HAD BECOME NECESSARY. I WENT ABOUT DOING A FEW OTHER NON-FLYING DUTIES IN PREPARATION FOR ARR AND THOUGHT NOTHING ELSE OF THE RESTR UNTIL IT WAS BROUGHT TO OUR ATTN BY THE CHICAGO CTLR ASKING IF WE 'HAD BEARS AT 11000 FT.' I RESPONDED THAT WE DID; TO INDICATE THAT WAS OUR ASSIGNMENT; AND TOLD THE CTLR THAT WE WOULD AT THAT TIME BE UNABLE TO COMPLY WITH THE RESTR. THE CAPT BEGAN THE DSCNT AND CROSSED BEARS AT APPROX 11500 FT. HE HAD MENTIONED A FEW TIMES THAT LEG HOW TIRED HE WAS AND HAD APPARENTLY FORGOTTEN ABOUT THE XING RESTR. I HAD DONE MY BEST TO MONITOR HIS FLYING AS A RESULT OF HIS SAYING HE WAS TIRED BUT EVEN IN MY INCREASED ALERTNESS COULD NOT WATCH HIM CLOSELY ENOUGH WHILE DOING THE OTHER DSCNT DUTIES TO NOTICE HE HAD NOT STARTED DOWN. THE CAPT PROCEEDED TO DISCONTINUE FLYING WITH A FATIGUE CALL WHEN WE REACHED CHICAGO.