B757-200 CREW MEMBER WAS OUT OF HIS SEAT WITHOUT THE OTHER CREW MEMBER BEING ON OXYGEN.

Narrative

I WAS CAPT ON AN INTL FLT FROM THE LOWER UNITED STATES TO ANCHORAGE. WHILE IN CRUISE AT FL390; THE FAA INTL RTE INSPECTOR LEFT THE FLT DECK FOR PHYSIOLOGICAL NEEDS. I LEFT MY SEAT TO SECURE THE FLT DECK DOOR DURING HIS ABSENCE AND SAT BACK DOWN. WHEN THE INSPECTOR RETURNED; THE FO LEFT HIS SEAT TO OPEN THE DOOR AND RESECURE THE DOOR. HE ALSO IMMEDIATELY SAT DOWN. DURING THE 20-30 SECONDS EACH OF US WAS ABSENT FROM OUR RESPECTIVE DUTY STATIONS; IN ORDER TO ENSURE THE SECURITY OF THE FLT DECK DOOR; NEITHER THE FO NOR I DONNED OUR OXYGEN MASKS. THIS WAS AN OVERSIGHT ON OUR PARTS; ESPECIALLY MINE AS CAPT. THE REG STATES 'SHOULD 1 CREW MEMBER LEAVE THEIR DUTY STATION; THE OTHER WILL WEAR AND USE OXYGEN TILL THE RETURN OF THE ABSENT CREW MEMBER TO THEIR DUTY STATION.' ALTHOUGH FATIGUE MAY HAVE PLAYED A PART IN MY OVERSIGHT (I HAD FLOWN A RED EYE FROM ANCHORAGE TO THE LOWER UNITED STATES THE PREVIOUS NIGHT; AND THE OCCURRENCE WAS AGAIN LATE ACCORDING TO MY BODY CLOCK); I AM STILL RESPONSIBLE TO ENSURE FAR AND FLT OPS DISCIPLINE IS MAINTAINED DURING ALL PHASES OF ALL FLTS.