AN ACR A320 FLC GETS DISTRACTED AND CROSSES A NAV FIX TOO HIGH. THE FLC BLAMES THE DISTRACTIONS AND CIRCADIAN RHYTHM UPSET.

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LATE FLT TO PHL FROM LAS. WE HAD BEEN IN CRUISE FLT AT FL370 FOR ABOUT 3 HRS WHEN INSTRUCTED FLY DIRECTLY TO THE LRP VORTAC. A SUBSEQUENT COM INSTRUCTED US TO CROSS 40 DME W OF LRP AT FL250. I PROGRAMMED THE XING RESTR INTO THE FMC AND ALLOWED THE COMPUTER TO CALCULATE A VERT DSCNT PATH. AT THIS POINT WE HAD BEEN GIVEN NO SPD RESTRS AND I WAS NOT ANTICIPATING ONE DUE TO THE TIME OF OUR ARR. EVENTUALLY; THE AIRPLANE CAPTURED THE COMPUTED VERT PATH AND IT INITIATED A 'MANAGED' DSCNT. WE RPTED LEAVING FL370 TO CTR AND HE ACKNOWLEDGED OUR COM. A FEW MINS LATER HE CALLED US BACK AND TOLD US TO REDUCE SPD TO 300 KIAS FOR TFC WE WERE FOLLOWING INTO PHL. I COMPLIED AND MADE A MENTAL NOTE THAT WE'D NOW BE HIGH ON DSCNT PROFILE; SO I SWITCHED DSCNT MODES FROM 'MANAGED' TO 'OPEN' (IDLE). AS WE WERE STILL ABOVE THE RECOMMENDED SPD BRAKE DEPLOYMENT ALT (THIS RECOMMENDED ALT IS NOT LIMITING); I ELECTED TO WAIT A FEW MINS BEFORE EXTENDING THEM AND THEREBY GIVE OUR PAX A SMOOTHER RIDE. AROUND THIS TIME THE COCKPIT BECAME FAIRLY BUSY (THE CAPT JOINED ME ON COM #1 FROM HAVING BEEN OFF GETTING ATIS AND ARR GATE INFO); WE DISCUSSED THE WX AT PHL AND THE APCH I WOULD BE FLYING. AT ONE POINT A FLT ATTENDANT ENTERED THE COCKPIT TO REQUEST SOME SPECIAL SVC ITEM. NEEDLESS TO SAY; I ENDED UP MISSING MY XING BY NEARLY 900 FT. BY THE TIME I EXTENDED THE SPD BRAKES IT WAS TOO LATE TO REJOIN THE VERT PROFILE WITHOUT EXCEEDING MY ASSIGNED SPD. WHAT I'VE LEARNED IS; WITH THE GREAT NUMBER OF LATE FLTS MY COMPANY FLIES; IT IS NECESSARY TO BE ESPECIALLY VIGILANT TO CRITICAL FLT ELEMENTS SUCH AS ALTS. THIS IS PARTICULARLY IMPORTANT ON THE FIRST DAY OF A TRIP (AS THIS WAS) WHEN ONE IS STILL IN THE PROCESS OF ADAPTING ONE'S BODY CLOCK TO AN ALIEN CIRCADIAN RHYTHM.