HDG TRACK ALTDEV ALT EXCURSION.

Narrative

I MADE THE TKOF FROM CASPER; WY. WE WERE CLRED TO FLY A 265 DEG HDG; PROCEED DIRECT TO BOISE; ID; BEFORE REACHING FL350. APPROX 250 NM E OF BOISE; AT FL350; WE RECEIVED A 260 DEG HDG AS A COURSE CORRECTION. I HAD THE AUTOPLT COUPLED AND MADE A HDG CHANGE.I THOUGHT I RETURNED THE TURN KNOB TO THE CTR DETENT. I WAS DISCUSSING SOMETHING WITH THE FE AND CAPT -- THE NEXT THING I RESPONDED TO WAS THE CAPT TELLING ME WE WERE ALMOST TO A 180 DEG HDG AND STILL TURNING. I IMMEDIATELY TURNED TO A 260 DEG HDG. WE WERE STILL CORRECTED BACK IN THE CORRECTING TURN WHEN CTR CALLED AND QUERIED US ON OUR POS. THE CTR DID NOT INDICATE ANY NEAR MISS OR OTHER TFC AS A CONFLICT. THE ONLY REASON THAT I CAN THINK I DID THIS IS FATIGUE. ALTHOUGH THIS WAS MY FIRST FLT AND I LAID DOWN IN TIME TO GET PLENTY OF SLEEP; I DIDN'T GET TO SLEEP UNTIL ABOUT 1 1/2 HRS BEFORE I HAD TO GET UP. BOTH THE OTHER CREWMEMBERS HAD BEEN UP ALL NIGHT FLYING; BUT WERE IN CREW REST PRIOR. SUPPLEMENTAL INFO FROM ACN 272054: A CONTRIBUTING FACTOR TO THIS SIT WAS THAT WE HAD BEEN FLYING ALL NIGHT AND WERE ALL VERY TIRED AT THE TIME. AS THE FO ATTEMPTED TO CORRECT BACK TO THE CORRECT HDG; HE LET THE ACFT CLB 450 FT.