FLC OF ACR LGT ACFT OVERSHOT CLB CLRNC ALT DUE TO ERRONEOUSLY SET ALT ALERTER.

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DURING CLBOUT FROM DAYTON INTL TO A CRUISE ALT OF 22000 FT MSL ENRTE TO FLINT; MI. ZOB INSTRUCTED US TO MAINTAIN FLT LEVEL 210 FOR XING TFC. I REPEATED THE CLRNC TO MAINTAIN 210 AND I BELIEVE I CHANGED THE ALT ALERT SYS FROM 22000 TO 21000. SHORTLY AFTER ATC'S AMENDED CLRNC THE FE HANDED TO US THE LNDG DATA CARD AS I STARTED TO ASK HIM WHAT THE LNDG NUMBERS WERE BECAUSE OF DIFFICULTY READING HIS WRITING. ATC CALLED AND ASKED US TO VERIFY OUR ALT I LOOKED UP AND SAW THAT WE WERE AT 22000 FT LEVEL AND THAT THE ALT ALERT SYS WAS READING 22000 FT. ATC ALSO ADVISED US TO RETURN TO 21000 FT BECAUSE OF AN ACFT AT 2 O'CLOCK AND 8 MI WHICH WAS OUR XING TFC. I ASKED THE CAPT WHAT HAD HAPPENED AND HE SAID HE DID NOT KNOW FOR SURE WHETHER WE HAD BEEN ASSIGNED 21000 OR 22000 FOR AN ALT. CONTRIBUTING FACTORS I BELIEVE TO THE ALT BUST ARE THAT WE HAD BEEN AWAKE ALL NIGHT FLYING AND A RECENT LAYOFF HAS REQUIRED CREWS TO FLY 23 NIGHTS A MONTH. FATIGUE HAS BECOME A VERY LARGE FACTOR AT OUR COMPANY CONCERNING SAFETY. THE FAA NEEDS TO MAKE CHANGES CONCERNING THE REST AND DUTY TIME RULES TO KEEP COMPANY'S SUCH AS OURS FROM ABUSING THE FLT CREWS IN THIS MANNER. THESE 23 NIGHTS A MONTH CONSIST OF DUTY TIMES RANGING FROM 10-14 HRS A NIGHT AND SOMETIMES 16 HRS. NASA IS INTERESTED IN SAFETY. PLEASE; FOR OUR SAFETY AND THE PAX OF OTHER ACFT WHO SHARE THE SAME SKIES WITH US; MAKE RECOMMENDATIONS TO THE GOV TO CHANGE THESE WORK RULES BEFORE A MAJOR CATASTROPHE TAKES PLACE.