A DC9 CREW; ON DSCNT NEAR LEX; OVERSHOT ASSIGNED ALT.

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DSCNT OUT OF FL330 TO CROSS 30 MI SW OF HYK AT FL290. AT APPROX FL295; ATC ISSUED A NEW RTE CLRNC. CAPT FLYING (ON AUTOPLT) AS FO PROGRAMMED NEW RTE INTO RNAV. I BECAME PREOCCUPIED WITH WATCHING FO TO SEE IF IT WAS PROGRAMMED CORRECTLY. ALTDEV CHIME OCCURRED AT FL287; AUTOPLT IMMEDIATELY DISCONNECTED AND CLB INITIATED BACK TO FL290. ACFT HAD DSNDED TO A MINIMUM ALT OF FL286. LEVEL AT FL290; ATC INQUIRED ABOUT OUR ALT. WE RESPONDED WE WERE LEVEL AT FL290. I DON'T BELIEVE ANY CONFLICT EXISTED FOR 2 REASONS. ATC CTLR SOUNDED VERY CALM AND DISINTERESTED AND APPROX 15 SECONDS LATER HE ISSUED DSCNT CLRNC TO FL280. THE MOST INFLUENTIAL HUMAN FACTOR WOULD BE FATIGUE; AS THIS WAS AFTER XA00 AND NO MATTER HOW MUCH REST YOU HAD THE DAY BEFORE; IT IS VERY TIRING FLYING THE BACK SIDE OF THE CLOCK. ALSO CONTRIBUTING IS THAT THE COMPANY ONLY HAS ABOUT 12 EFIS EQUIPPED ACFT OUT OF FLEET OF XO DC9'S. I ONLY FLY EFIS ACFT APPROX 1-2 TIMES PER YR AND ALL DISPLAYS AND CTLS ARE VERY DIFFERENT REQUIRING MUCH MORE ATTN TO ENSURE RIGHT BUTTONS ARE PUSHED. ACFT FLEET SHOULD BE MORE STANDARD!