APCHING EDFF AFTER A VERY LONG FLT; FLC WAS INSTRUCTED TO DSND TO 8000 FT AND OVERSHOT THE ALT. BEFORE A CORRECTION COULD BE MADE; THE CTLR ISSUED A CLRNC TO 7000 FT.

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CREW WAS TIRED AFTER FLT ARRIVED OVER RUD NDB TO HOLD FOR SPACING INTO EDDF. AFTER SEVERAL DSCNTS; RADAR VECTORS AND AN ADDITIONAL DSCNT CLRNC WAS ISSUED FOR OUR FLT. DURING A DSCNT TO 8000 FT; OUR ATTN WAS MOMENTARILY DISTR AND NO CREW MEMBER NOTICED THE 'ALT ARMED' LIGHT (WHICH IS NEEDED FOR THE APCH IN USE TO CAPTURE THE SELECTED ALT) WAS NOT ILLUMINATED. ACFT DSNDED APPROX 300 FT BELOW THE ASSIGNED ALT. A GENTLE BUT FIRM RECOVERY WAS IMMEDIATELY INITIATED. THE CTLR CALLED FOR OUR FLT TO CONTINUE DSCNT TO 7000 FT. NOTHING ELSE WAS SAID ABOUT THE ALTDEV BY THE CTLR. AT LEAST 1 CREW MEMBER HAD BEEN AWAKE FOR ABOUT 18 HRS. ALL WERE TIRED. IT'S OBVIOUS THAT TIRED OR FATIGUED CREWS ARE NOT AS SHARP AS THOSE THAT ARE MORE RESTED; BUT CONSTANT ATTN TO DETAILS ARE THE ONLY ANSWER TO AVOIDING SIMILAR EVENTS FROM RECURRING. IT WAS THE PF'S RESPONSIBILITY TO MONITOR THE ACFT AT ALL TIMES; AND THE MOMENTARY LAPSE OF ATTN ALLOWED THE ACFT TO DSND BELOW THE ASSIGNED ALT. IT WAS; HOWEVER; THE CRM TRAINING AND TOTAL CREW COORD THAT IDENTED THE ERROR; AND INITIATED AN IMMEDIATE RESPONSE TO THE PROB.