B757 CREW HAD ALT UNDERSHOOT.

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JUST PRIOR TO LEAVING FL370 FOR FL230; THE FO HAD TO MAKE A QUICK TRIP TO THE BATHROOM. WE WERE CLRED TO CROSS HARTY INTXN AT AND MAINTAIN FL230. A STANDARD CLRNC FOR JFK ARRS. DONE IT A HUNDRED TIMES. I PUT ON OXYGEN MASK AND SET UP RADIO PANEL. SUN HAS JUST COME UP AND IT IS A BEAUTIFUL MORNING; ALTHOUGH VERY BRIGHT WITH SUN IN EYES. FMC STARTS ACFT NOSE DOWN AT THE PROPER DSCNT POINT FOR FL230. I CALL OUT OF FL370. COMPUTER SHOWS ACFT WILL CROSS HARTY INTXN 1 MI BEFORE HARTY. SPD IS GOOD. AT ABOUT FL340 FO RETURNS TO COCKPIT; GETS IN SEAT AND HOOKS UP. FLT ATTENDANT KNOCKS ON DOOR; HAS COFFEE (NOT ASKED FOR) AND WANTS TO KNOW LNDG TIME. FO ASSUMES FLYING DUTIES. COMPUTER SHOWS ACFT RIGHT ON PROPER DSCNT PROFILE. I'M PUTTING OXYGEN MASK AWAY; FO IS GETTING HIS COFFEE; AND ACFT IS AT APPROX FL280. ATC ASKS IF WE'RE GOING TO MAKE THE HARTY XING RESTR AT FL230. I WONDER WHO HE ASKED; AND CHKED THE FMC DISPLAY. COULDN'T BELIEVE MY EYES. DSCNT RATE HAD CHANGED AND IT WAS OBVIOUS WE COULDN'T CROSS HARTY AT FL230. I TOLD THE FO TO TELL CTR WE COULDN'T MAKE FL230; ASSUMED THE FLYING DUTIES; TOOK ACFT OFF AUTOPLT; PULLED SPD BRAKES; AND ATTEMPTED TO GET AS LOW AS POSSIBLE TO FL230. ONCE WE SAID WE COULDN'T MAKE IT; THE CTLR INDICATED THAT HE DIDN'T HAVE OUR ALT (IT HAD CROSSED OUT?) BUT THERE WAS NOT A PROB AND WOULD WE PLEASE DSND NOW SO AS TO CROSS THE NEXT INTXN; LENDY; AT FL190. I BELIEVED WE CROSSED HARTY AT FL240. WE CROSSED LENDY AT FL190 AND REST OF FLT WAS ROUTINE. WHAT CAUSED THE PROB? TECHNICALLY; I DON'T KNOW. FMC WAS RIGHT ON DSCNT PATH AT APPROX FL330. AND THEN THE RATE OF DSCNT CHANGED. HUMAN FACTORS WISE; MY FAILURE TO CONTINUOUSLY MONITOR THE DSCNT RATE DURING ALL THE DISTRACTIONS OF FO RETURNING; UNWANTED COFFEE; SUN IN EYES; OXYGEN MASK STOWAGE WAS THE REAL CULPRIT HERE. I ASSUMED THE FO WAS MONITORING; AS HE HAD TAKEN OVER; BUT HE WAS JUST AS COMPLACENT AS I WAS. ONCE AWARE WE COULDN'T MAKE THE ALT; HE SEEMED DEVASTATED. CORRECTING THE SIT. FROM NOW ON; I WILL BRIEF MY CREW THAT ALL 'POTTY' BREAKS MUST BE DONE BEFORE DSCNT FROM CRUISE. ONCE DSNDING; I WILL NEVER AGAIN GIVE THE COMPUTER CREDIT FOR BRAINS. I WILL CONTINUOUSLY MONITOR; EVEN WHILE DOING OTHER COCKPIT DUTIES. AND I WILL MAKE SURE MY FO DOES AS WELL. THANK GOD; THIS WAS EARLY IN THE MORNING WITH VERY; VERY LITTLE TFC IN THE AREA.