B737 FLC MISSED XING ALT DURING STAR ARR DSCNT DUE TO COMPLACENCY AND FATIGUE.

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ON THE ARVIN 3 ARR TO PHX SOMETIME PRIOR TO BUT VOR AT FL270 WE WERE CLRED TO CROSS SCOLE INTXN AT FL250. RESET ALT ALERTED; CAPT ACKNOWLEDGED; AND VERIFIED RESTR IN FMC AS WELL. CAPT STARTED DSCNT LATE (1-2 MI FROM SCOLE) AND CTLR CALLED TO VERIFY OUR ASSIGNMENT. WE TOLD HER WE WERE IN THE DSCNT AND SHE HANDED US OFF TO THE NEXT SECTOR. I CANNOT SAY FOR CERTAIN EXACTLY HOW HIGH WE WERE XING SCOLE; BUT I'M SURE IT WAS AT LEAST 1000 FT. THIS EVENT IS VERY TROUBLESOME TO ME. THERE WERE NO DISTRS (IE; FLT ATTENDANTS) IN THE COCKPIT. NEITHER THE CAPT NOT I WERE INVOLVED WITH OTHER TASKS; AND THE WORK AT THE TIME WAS MINIMAL. WE WERE OBVIOUSLY BOTH VERY PREOCCUPIED. THESE ARE SOME POSSIBLE FACTORS: FATIGUE -- THIS WAS THE LAST LEG OF THE LAST DAY OF A PRODUCTIVE PAIRING. WE HAD BEEN ON DUTY FOR OVER 10.5 HRS AND HAD 2 ACFT SWAPS IN THE LAST 3 LEGS. NEITHER OF US HAD EATEN IN AT LEAST 7 HRS. I CANNOT SPEAK FOR THE CAPT; BUT I HAD SLEPT POORLY THE NIGHT BEFORE. ANXIETY -- AT EVERY TURN THROUGHOUT THE DAY (3 DAYS) WE RECEIVED MORE DISMAL NEWS ABOUT OUR NATION AND OUR INDUSTRY. AS WE FLEW E TOWARD BWI; WE COULD SEE MIL MANEUVERS (FLARES AND BOMB FLASHES) BY GILA BEND. PREPARATIONS FOR WAR? AT THE TIME OF OUR INCIDENT WE WERE BOTH DEEP IN OUR OWN THOUGHTS DESPITE OUR BEST INTENTIONS.