ALTDEV ALT OVERSHOT.

Narrative

A LEVELOFF BELOW ASSIGNED ALT OCCURRED BECAUSE OF A QNH ALTIMETER SETTING ABOVE FL180. THE ALTIMETER WAS EITHER WRONGLY SET ON CLBOUT OR INAPPROPRIATELY SET DURING DSCNT. THE CLB TO CRUISE ALT WAS DELAYED BY ATC AT 10000 FT MSL AND INTERMEDIATE ALTS. PRIOR TO DSCNT; AN EXTENSIVE AND THOROUGH BRIEFING FOR A CAT III APCH FOR RWY 4R WAS COMPLETED. IT WAS ANTICIPATED THAT THE FLT WOULD CROSS PROVIDENCE ON THE NORWICH 2 ARR AT 11000 FT. I AM NOT CERTAIN WHEN THE INCORRECT SETTING OF THE ALTIMETER OCCURRED; ON CLBOUT OR DURING DSCNT. I BELIEVE FATIGUE MAY HAVE BEEN THE CAUSE IN ADDITION TO SOME MINOR DISTRS. THE CREW IS BASED ON THE WEST COAST. AFTER A 24- HR REST; BEGAN DUTY FOR 3 LEGS. ALTHOUGH PROPER REST WAS PLANNED; I EXPERIENCED INTERRUPTIONS OF SLEEP DUE TO CONVENTION AT THE HOTEL. AT THE START OF THE DUTY PERIOD; I FELT ALERT AND THE FIRST 2 LEGS WERE NORMAL AND UNEVENTFUL. THE 3RD LEG APPEARED TO BE FLOWN ACCORDING TO ALL SOP'S AND A NON-EVENT EXCEPT FOR THE INCORRECT ALTIMETER SETTING. FOLLOWING THE FLT; I FELT SOME PHYSICAL EXHAUSTION AND THEREFORE THINKING I MAY HAVE BEEN LESS ALERT THAN NORMAL. IT IS DIFFICULT TO REST WHEN THE DUTY PERIODS ARE CHANGED AS THEY ARE IN THIS SEQUENCE. VERIFICATION OF ASSIGNED ALT AND PROPER ALTIMETER SETTING WILL BE XCHKED TO PREVENT ANOTHER OCCURRENCE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR FLIES THE B-757 FOR A MAJOR UNITED STATES ACR ON DOMESTIC RTES. THE CAPT WONDERS IF HE EVER SET HIS ALTIMETER ON CLBOUT. HE DOES NOT KNOW WHEN THIS ANOMALY OCCURRED; AND IT WAS NOT DETECTED UNTIL ARTCC SAID 'CHK YOUR ALT.' THE WX WAS VARIABLE AT JFK; AND THE CAPT WAS INDECISIVE ABOUT WHETHER A CAT III APCH WOULD HAVE TO BE FLOWN. HE NOW WISHES THAT HE HAD NOT DITHERED ABOUT THIS POINT AND JUST PLANNED TO FLY THE CAT III PROCS WITHOUT QUESTION. THE CAPT SET; OR FAILED TO SET; HIS OWN ALTIMETER AND THE FO FAILED TO CHK IT. THE NOISE IN THE HOTEL WAS BAD ENOUGH THAT THE FO HAD TO CHANGE ROOMS. THE CAPT DOES NOT LIKE ALL NIGHT TRIPS.