HDG TRACK ALTDEV ALT EXCURSION.

Narrative

WE WERE ON 4TH DAY OF 4 DAY TRIP. MINIMUM REST (LEGAL). LEFT HOTEL THAT MORNING AT AM40 SO BOTH OF US WERE VERY TIRED. WHILE ON A HDG OF 140 DEGS AND ALT OF 22000 FT; I HEARD CTR GIVE US INSTRUCTIONS TO GO TO 180. THE FO ANSWERED; 'ROGER; FLT X; 180.' I UNDERSTOOD THIS AS FLT LEVEL 180 NOT HDG 180. I CHANGED THE ALT SELECT CTL TO 18000 FT AND BEGAN A SLOW DSCNT. A SHORT TIME LATER; CTR CALLED BACK AND SAID HE SHOWED US 700 FT LOW. I ANSWERED THAT HE GAVE US A DSCNT TO FLT LEVEL 180. HE REPLIED NO; I MEANT TURN TO 180 DEGS. I ACKNOWLEDGED AND RETURNED TO 22000 FT AND TURNED TO A HDG OF 180 DEGS. NO CONFLICT WAS NOTED. SUPPLEMENTAL INFO FROM ACN 271950: CRUISING AT 22000 FT 140 DEG HDG. ATC CLRED US TO 180 DEG HDG. I READ BACK 180 DEG HDG. CAPT (FLYING) CHANGED ALT. PRESELECT TO 18000 FT. SLOW DSCNT WAS STARTED TO 18000 FT; ACFT ON 140 DEG HDG. DSCNT WAS NOT NOTICED BY ME (PNF). CAPT TOOK UPON HIMSELF TO CHANGE ALT. PRESELECT WHICH WAS AGAINST COMPANY PROCS. (PNF CHANGES ALT PRESELECT.) NO CONFLICT NOTED.