FLC FORGOT TO TURN ON THE SEAT BELT SIGN.

Narrative

FO COMMENCED A GRADUAL DSCNT IN ORDER TO CROSS NIKLS INTXN (E OF COLUMBUS) AT 11000. THE SEAT BELT SIGN DID NOT GET TURNED ON UNTIL FLAP EXTENSION WHILE DOWNWIND. THE AUTO POS OF THE SWITCH ILLUMINATED ALL SEAT BELT SIGNS WHEN THE FLAPS CAME TO POS #1. THERE WAS NO ASSISTANCE OR CALLS OVER THE INTERCOM FROM ANY OF THE FLT ATTENDANTS TO BACK US UP. IT WAS DURING MY SHUTDOWN FLOW AFTER BLOCK-IN AT COLUMBUS THAT I NOTICED THAT THE SWITCH WAS IN THE AUTO POS (RATHER THAN ON). I QUESTIONED THE FLT ATTENDANTS AS TO WHY THEY DID NOT CALL US AS A REMINDER -- THE #1 FLT ATTENDANT RESPONDED THAT HE THOUGHT THERE WAS A MALFUNCTION THAT CAUSED THE SIGNS NOT TO ILLUMINATE. THE CABIN WAS FULLY PREPARED FOR LNDG AS WE MADE A 10000 FT AGL CALL OVER THE PA SYS DURING DSCNT PER ACR -- SOP. I ATTRIBUTE THIS INCIDENT TO FLC FATIGUE. THIS WAS DAY 2 OF A 3 DAY TRIP. THE PHOENIX DOMICILED CREW INTO A WAKEUP TIME IN COLUMBUS AT APPROX XE45 LCL (XB45 PHOENIX TIME). I FIND IT EXTREMELY DIFFICULT TO GET TO SLEEP AT XV:PM (XS:PM BODY CLOCK) IN ORDER TO NOT FEEL FATIGUED THE NEXT DAY.