CAPT OF AN MLG CANCELED THE VNAV MODE FOR THE FMS CAUSING THE ACFT TO OVERSHOOT THE DSCNT ASSIGNED ALT.

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WE WERE INBOUND TO ATL ON THE ROME 8 ARR. OUR CLRNC WAS TO CROSS ERLIN INTXN AT 13000 FT THEN DSND TO 9000 FT. THE ACFT WAS IN VNAV DSCNT AND FMS WAS SET TO CROSS ERLIN AT 250 KTS AND 13000 FT. THE CAPT SELECTED 9000 FT IN ALT SELECT WINDOW. THE ACFT WAS SET TO CROSS ERLIN AT 13000 FT AND WE WOULD HAVE COMPLIED WITH THE ATC CLRNC. SOMEWHERE PRIOR TO 13000 FT THE CAPT CANCELED VNAV AND SELECTED IAS ON THE AUTOPLT. I DIDN'T SEE THIS HAPPEN AS I WAS GETTING GATE INFO OFF THE ACARS PRINTER. NOW THE ALT RESTR AT ERLIN WAS CANCELED AND THE ACFT WOULD GO DIRECT TO 9000 FT. AS A RESULT WE CROSSED ERLIN AT APPROX 11000 FT. AS IT TURNED OUT THE CAPT SAID HE DIDN'T REALIZE WE NEEDED TO CROSS ERLIN AT 13000 FT AND THOUGHT WE WERE CLRED DIRECTLY TO 9000 FT. I THINK FATIGUE HAD SOMETHING TO DO WITH THIS INCIDENT BECAUSE WE HAD AN XA00 AM RPT AT THE ARPT AND WE WERE BOTH TIRED THAT MORNING. THIS LED TO THE BREAKDOWN IN CREW COORD BECAUSE WE WERE NOT AGGRESSIVE IN MAKING SURE WE BOTH UNDERSTOOD THE CLRNC.