FO OF A CANADAIR CL600 DSNDED BELOW ASSIGNED TRANSITION ALT DURING VECTOR TO FINAL APCH.

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APCHING LAS JUST AFTER XA00 ON MAY/XA/00; WE WERE VECTORED TOWARD TIFFY; AN OUTER APCH FIX FOR RWY 25L; CLRED FOR THE ILS AND INSTRUCTED TO MAINTAIN 8000 FT UNTIL XING TIFFY. SUBSEQUENTLY; WE RPTED THE RWY IN SIGHT AND WERE CLRED FOR A VISUAL TO RWY 25L. MY FO; WHO WAS FLYING; CONTINUED A SLIGHT DSCNT. WE WERE SLIGHTLY BELOW 8000 FT AS WE CROSSED ABEAM THE TIFFY FIX; AS HE WAS ANGLING MORE DIRECTLY TO THE RWY BY NOW. THE CTLR THEN QUERIED US AS TO WHY WE HAD DSNDED BELOW 8000 FT. I'M SUBMITTING BECAUSE I'M NOT SURE IF WE WERE IN VIOLATION OF AIRSPACE. WE SHOULD HAVE CONTINUED TO REMAIN WITHIN CLASS B AIRSPACE; BY MY RECKONING; BASED ON THE DME'S OF THE SEGMENTS IN RELATION TO TIFFY. HOWEVER; AT THAT POINT IT WAS PAST XB00 BY OUR BODY CLOCKS. WE HAD STARTED THE DAY IN BGR WITH A LATE EVENING DEP; BUT HADN'T BEEN ABLE TO NAP THAT AFTERNOON; AS PLANNED; DUE TO AN AEROBATIC SHOW PERFORMING RIGHT OVER THE HOTEL DURING AIR SHOW THERE AND THE HOTEL REQUESTING US TO DEPART 1 HR EARLIER THAN PLANNED DUE TO ROADS CLOGGED WITH AIR SHOW TFC. WE WERE BOTH FEELING THE EFFECTS OF NEEDING SLEEP AND POSSIBLY DID NOT RECOGNIZE THE NECESSITY TO MAINTAIN THE 8000 FT (IF; INDEED; IT WAS NECESSARY) AFTER BEING CLRED FOR THE VISUAL.