FLC OF AN MLG FAILED TO START DSCNT TO COMPLY WITH ATC XING RESTR RESULTING IN ATC INTERVENTION AND A NEW XING FIX.

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FLYING ENRTE AT 31000 FT FROM NY LAGUARDIA TO CLEVELAND HOPKINS ARPT; WE PASSED PSB VOR AND WERE CLRED BY CTR TO GO DIRECT TO CXR VOR. SHORTLY THEREAFTER; WE WERE CLRED BY CTR TO CROSS 40 MI E OF CXR VOR AT 24000 FT. I SET 24000 FT INTO THE FLT GUIDANCE COMPUTER; BUT SINCE A DSCNT TO MEET THIS RESTR WAS NOT REQUIRED FOR A WHILE; I DID NOT START A DSCNT AT THAT TIME. LATER; AT ABOUT 45 MI E OF CXR VOR; CTR CALLED AND ASKED IF WE WOULD BE ABLE TO MAKE THE RESTR. WE REALIZED WE WOULD NOT BE ABLE TO AND ASKED THEM FOR ANOTHER. CTR THEN CLRED US TO DSND TO 24000 FT CLOSER TO CXR. FATIGUE WAS THE PROB HERE. WE WERE BOTH SO TIRED THAT WE COMPLETELY FORGOT TO START THE DSCNT. WE HAD ALREADY BEEN FLYING FOR A COUPLE OF DAYS. THIS LGA-CLE FLT WAS THE THIRD FLT OF THE DAY. THIS DAY WE WERE SCHEDULED FOR 8 HRS ACTUAL FLYING TIME; WHICH IS THE LEGAL MAX. WE ENCOUNTERED SUBSTANTIAL DELAYS GETTING INTO AND OUT OF LGA DUE TO WX AND TFC IN THE NY AREA. THEREFORE; BY THE TIME THIS PROB OCCURRED; WE WERE QUITE TIRED. OUT ACTUAL BLOCK TO BLOCK FLT TIME FOR THE DAY ENDING AT CLE WAS 9 HRS 26 MINS; WHICH IS WELL OVER THE 8 HR LEGAL MAX SCHEDULE TIME FOR DOMESTIC OPS. AIRLINE COMPANIES ROUTINELY SCHEDULE PLTS TO FLY AT THE LEGAL LIMITS; KNOWING THAT THE PLTS WILL PROBABLY HAVE TO FLY A LOT MORE WHEN GOING INTO DELAY PRONE AREAS LIKE LGA. THIS IS NOT A SAFE PRACTICE ON THEIR PART.