FO OF A CPR EXECUTIVE JET STARTED DSCNT FROM APCH TRANSITION ALT DUE TO MISTAKENLY TAKING ANOTHER ACFT'S CLRNC.

Narrative

I BELIEVE THIS IS ANOTHER EXAMPLE OF AN OCCURRENCE DUE TO FATIGUE. I WAS IN LAS VEGAS; PLANNING TO GO OUT THE FOLLOWING DAY. WE WERE THERE FOR SCHEDULED MAINT. I GOT A CALL AT XA00 FROM COMPANY INFORMING ME OF A TRIP THAT EVENING FROM LAS TO PDX ON TO COS. GOT TO ARPT EARLY TO CHK STATUS OF THE PLANE AND TO FLT PLAN. I DISCOVERED A LEG FROM PDX TO SFO HAD BEEN ADDED. I CHKED TO SEE IF IT WAS WITHIN OUR DUTY TIME OF 14 HRS. IT WAS. I HAD BEEN AWAKE SINCE XO00. FIRST LEG (PDX) WENT WELL. DEP FROM PDX WAS DELAYED 50 MINS DUE TO LATE PAX. I HAD THE SIC FLYING THIS LEG. HE IS TYPED AND HAS EXPERIENCE IN THE PLANE. HE IS A PIC ON OTHER COMPANY ACFT. ENRTE WE BOTH EXHIBITED SOME WEARINESS -- YAWNING; ETC. BY THE TIME WE GOT TO SFO; I WAS TIRED; AS I HAD BEEN UP BETTER THAN 18 HRS ALREADY. WE WERE BEING VECTORED FOR A R BASE ENTRY TO RWY 28R FOR A VISUAL APCH. OUR ASSIGNED ALT WAS 6000 FT MSL. THE NEXT THING I RECALL WAS OBSERVING THE COPLT INITIATING A DSCNT. I HAD HEARD THE CTLR BUT DID NOT CLRLY HEAR HIS COMMENTS AS IF I WAS IN A MOMENTARY STATE OF SLEEP. I SNAPPED TO ATTN AND ASKED THE SIC IF HE HAD BEEN GIVEN A DSCNT CLRNC. HE STATED THAT HE HEARD THE CTLR CLR US FOR THE VISUAL APCH. WE HAD DSNDED AROUND 1000 FT AND THE CTLR IMMEDIATELY ISSUED A COMMAND TO CLB BACK TO 6000 FT AND TO TURN TO A HDG; WHICH I DO NOT RECALL. UPON QUESTIONING THE OTHER PLT AGAIN; HE STATED HE HEARD A CLRNC FOR A VISUAL DSCNT. I'M NOT SURE FOR I WAS IN THE TWILIGHT ZONE MOMENTARILY. THE CTLR SOUNDED AS THOUGH HE WAS WORKING A LOT OF TFC FOR THAT TIME OF MORNING; AND POSSIBLY WORKING MORE THAN 1 STATION. WE WERE TURNED BACK AND CLRED TO TWR; WITHIN A FEW SECONDS. UPON CALLING TWR; WE WERE CLRED FOR THE VISUAL APCH AND LNDG; NOTHING ELSE WAS SAID. I'M NOT SURE HOW MUCH OF A TFC CONFLICT THERE WAS; IF THERE WAS ONE. I DID NOT SEE ANOTHER ACFT; EXCEPT ONE ON SHORT FINAL TO OUR RWY. WE LANDED; UNLOADED THIS PAX; FUELED; AND DETERMINED AN APPROPRIATE METHOD OF STAYING AWAKE. UPON COMPLETING THE MISSION; I HAD BEEN ON MY FEET 23 HRS. HOWEVER; I WAS WITHIN OUR DUTY LIMITS. I UNDERSTAND BETTER NOW THE COMPLAINTS OF THE ACR PLTS OVER THE ISSUE OF REST AND DUTY TIME. HOPE THIS MIGHT HELP SOMEONE ELSE OUT THERE FLYING THE FRACTIONALS OR OTHER RELATED INDUSTRIES.