FLC OF LGT CARGO OVERSHOT A XING RESTR ALT ON A STAR DUE TO FATIGUE.

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WE WERE ATTEMPTING TO MAKE A XING RESTR ON THE MIAMI HEATT 2 ARR. THE FO WAS FLYING AND HAD THE SPD BRAKES DEPLOYED; FOR RAPID DSCNT; TO MAKE THE XING RESTR OF 16000 FT. WHEN HE REACHED 16000 FT; I OBSERVED HE WAS NOT GOING TO LEVEL OFF. SO; I CALLED ALT. BUT WITH SPD BRAKES DEPLOYED; HE WAS NOT ABLE TO REACT IN TIME; AND THE ACFT WENT 400-500 FT BELOW LEVELOFF ALT OF 16000 FT. HE IMMEDIATELY RECOVERED TO 16000 FT WITHIN ABOUT 3-5 SECONDS. ZMA DID NOT SEE THE ALT EXCURSION; OR EITHER THEY SAW IT AND DID NOT SAY ANYTHING TO US. ALL 3 CREWMEMBERS WERE EXTREMELY TIRED. WE HAD BEEN UP ALL NIGHT; AND THIS WAS THE LAST LEG OF OUR TRIP. THE PREVIOUS DAY WE HAD A SHORTER REST PERIOD BECAUSE OF DELAYS AND ACFT MECHANICAL PROBS. AS YOU KNOW; WHEN FLYING ON THE BACK SIDE OF THE CLOCK; YOU BECOME MORE FATIGUED AND REACTION TIMES SLOW DOWN. THESE THINGS WILL OCCASIONALLY CONTINUE TO OCCUR. BUT ONE WAY TO PREVENT OR MINIMIZE THEM FROM OCCURRING IS TO CHANGE THE PART 121 DOMESTIC FLT AND DUTY TIME REGS FOR SCHEDULED NIGHT CARGO OPS. UNFORTUNATELY; THIS WILL NEVER HAPPEN; UNTIL THERE IS AN ACCIDENT THAT INVOLVES NUMEROUS FATALITIES OF CIVILIANS ON THE GND; SINCE NIGHT CARGO ACFT CARRY NO PAX. IT'S A SHAME THAT SOMETHING LIKE THIS WILL HAVE TO OCCUR BEFORE THE FAA WILL CHANGE THE REGS.