WHEN A B757 WAS UNABLE TO CLB TO THE ASSIGNED ALT DUE TO ITS HVY WT; THE FLC OBTAINED A CLRNC FOR A LOWER ALT. THE PF MISREAD THE ALTIMETER AND INITIATED CONFLICTING COMMANDS BY DISENGAGING THE AUTOTHROTTLES AND MANUALLY SELECTING CLB THRUST WHILE THE PNF WAS SELECTING THE FLT LEVEL CHANGE MODE ON THE AUTOPLT FOR A DSCNT BACK DOWN TO FL370. THE ACFT CONTINUED TO CLB. BOTH CREW MEMBERS RPT FATIGUE AS A CONTRIBUTING FACTOR.

Narrative

THE FLT HAD BEEN CLRED TO CLB TO FL390; DUE TO POOR CLB PERFORMANCE; PASSING APPROX FL375; CLRNC TO RETURN TO AND MAINTAIN FL370 WAS REQUESTED BY THE CREW AND RECEIVED FROM ZAB. AS THE ACFT WAS PASSING FL377; I WAS INSTRUCTED TO SELECT THE FLT LEVEL CHANGE MODE ON THE AUTOPLT. I DID SO; AND AT THE SAME TIME; ALSO SET A LOWER MACH NUMBER IN THE SPD WINDOW SINCE WE WERE VERY NEAR VMO; I DID NOT WANT TO SET OFF THE SPD WARNING SIREN. I WAS UNAWARE THAT THE CAPT HAD TURNED OFF THE AUTOTHROTTLE AND MANUALLY ADVANCED THE THROTTLES TO MAX CLB. APPARENTLY HE HAD MISREAD THE ALTIMETER. EVEN SO; INSTEAD OF DSNDING AS COMMANDED; THE AUTOPLT DID NOT; AND APPARENTLY ATTEMPTED TO MAINTAIN THE SET SPD OF MACH .82. AT APPROX FL379 TO FL380; I QUESTIONED WHY WE WERE STILL CLBING; AND SELECTED THE VERT SPD MODE ON THE AUTOPLT TO INITIATE THE DSCNT. THE CAPT IMMEDIATELY CONTACTED CTR. THE CTLR ACKNOWLEDGED AND THE ACFT WAS QUICKLY RETURNED TO FL370. CREW FATIGUE WAS UNDOUBTEDLY A FACTOR. THIS WAS THE THIRD LEG OF AN ALL NIGHT XCONTINENT FLT AT THAT PERIOD JUST BEFORE DAWN; WHERE THE EFFECTS OF FLYING ON THE 'BACKSIDE OF THE CLOCK' SEEM MOST SEVERE. SUPPLEMENTAL INFO FROM ACN 334680: THE ACFT NEEDED TO DSND TO FL370 AND I BELIEVED IT NEEDED TO CLB. I MADE THE SIT WORSE BY DISENGAGING THE AUTOTHROTTLES AND MANUALLY SELECTING CLB THRUST. AT THAT THRUST AND SPD THE ACFT COULD NOT DSND WITHOUT REDUCING PWR. AS THE AUTOTHROTTLES WERE DISENGAGED THE AUTOPLT COULD NOT CTL THRUST. DUE TO THE EARLY HR THERE WAS NO OTHER TFC. HAD I VERBALIZED WHAT I THOUGHT THE SIT WAS; THE FO WOULD HAVE CORRECTED ME.