TRACK DEV WHEN CLRNC IS CONFUSED AT THE END OF AN ALL NIGHT 11 HR DUTY DAY.

Narrative

CREW WAS BEING VECTORED BY CVG APCH AND THE FO RESPONDED VERBALLY TO CVG APCH TO MAKE A R TURN TO 320 DEGS (DOWNWIND FOR CVG RWY 18L). THE CAPT WAS FLYING AND THOUGHT THE INSTRUCTION WAS TO TURN TO HDG 230 DEGS. THE PNF FO BELIEVED THE TURN TO THE L WAS JUST A MOMENTARY TURN TO AVOID A CONVECTIVE BUILDUP BEFORE TURNING AS INSTRUCTED. BY THE TIME THE FO REALIZED THE CAPT HAD MISUNDERSTOOD THE VECTOR AND WAS IN THE PROCESS OF DISCUSSING THE WRONG TURN WITH THE CAPT; ATC CALLED US AND REDIRECTED US FOR OUR SEQUENCE INTO CVG. THE APCH CHKLIST WAS BEING CONDUCTED DURING THE WRONG TURN. THE CREW HAD BEEN ON DUTY SINCE XA40 THE PREVIOUS DAY (OVER 11 HRS WITH LITTLE REST). THIS IS A TYPICAL EXAMPLE OF HOW FATIGUE CAN CAUSE DRASTICALLY DETERIORATED PERFORMANCE IN EVEN THE SIMPLEST OF FLYING TASKS. I BELIEVE THE FAA SHOULD RE-EVALUATE REST RULES FOR CONTINUOUS DUTY; 'BACK SIDE OF THE CLOCK' OPS. I HAVE SEEN THIS TYPE OF SIT OCCUR MANY TIMES FOR THE SAME REASONS: CONTINUOUS DUTY; REDUCED REST; OR INADEQUATE REST.