CL65 FOLLOWS TCAS RA ADVISORY WHEN THEIR ALT ALERT IS SET AT 7000 VICE 8000 AS CLEARED.

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ON THE ARR INTO PHX ARPT; WE WERE GIVEN A SHORT HOLD. OUT OF THE HOLD WE WERE TOLD TO EXPECT RWY 26. A FEW MINS LATER WE WERE TOLD THERE WAS AN EMER ON RWY 26 AND TO CHANGE RWYS TO RWY 25L. WE WERE GIVEN A FREQ CHANGE AND THEN GIVEN AN ALT AND HDG. I THOUGHT THE ALT WAS 7000 FT AND DIALED IT INTO THE ALT SELECT. AT THIS TIME THE CAPT WAS FINISHING A CHKLIST; AND REPROGRAMMING THE FMS FOR THE NEW RWY. I NOTICED THAT WE WERE STILL IN NAV MODE ON THE FMS AND ASKED HIM TO VERIFY THE HDG WITH ATC. WE WERE VERY BUSY. AT THE SAME TIME WE NOTICED ANOTHER ACFT ON TCAS IN CLOSE PROX. WE WERE ALREADY DSNDING THROUGH 8000 FT AT 1.2 VSI. ALMOST SIMULTANEOUSLY; AT 7700 FT; WE GOT A TCAS ALERT; I DISCONNECTED THE AUTOPLT; INITIATED A CLB; AND ATC TOLD US TO CLB. THE WX IN PHX WAS OUT OF THE ORDINARY -- WINDY; TURBULENT AND RAINING. THERE WAS A LOT GOING ON IN THE COCKPIT; SEVERAL FREQ CHANGES AND THE CTLRS WERE BUSY. I POSSIBLY MISUNDERSTOOD A HDG OF 070 DEGS AND DSCNT TO 8000 FT AS 080 DEG HDG AND DSCNT TO 7000 FT. I THINK THIS IS A GOOD EXAMPLE OF A 'SYS' WORKING (TCAS; ATC; COCKPIT PROCS) AND ANOTHER NOT (UNREASONABLE FARS). WE SHOULD HAVE VERIFIED THE ALT AND HDG SOONER BUT WHEN THAT DIDN'T HAPPEN THE TCAS ALERTED US AND THEN ATC QUERIED US. AS A CREW I THINK WE WERE AS ALERT AS WE COULD HAVE BEEN; OUR WORKLOAD WAS AT THE MAX. THESE KIND OF MISTAKES HAPPEN AND THE CREW HAS TO TRY TO CATCH THEM EARLIER. I ALSO THINK THAT BURNOUT FROM FLYING TOO MANY HRS AND UNREASONABLE SCHEDULES WAS PROBABLY A CONTRIBUTING FACTOR IN THIS INCIDENT. MOST PLTS AT OUR AIRLINE FLY FAR TOO MANY HRS IN A DAY; WK; MONTH AND YR. THE MORE FATIGUED AND OVERWORKED PLTS YOU HAVE THE MORE INCIDENTS LIKE THIS WILL HAPPEN. THERE ARE NUMEROUS STUDIES DEMONSTRATING THE EFFECTS OF FATIGUE ON PERFORMANCE YET THE FAA CHOOSES TO IGNORE THEM. 16 HR DUTY DAYS; 8 DAYS OFF A MONTH; 8 HR REST PERIODS RESULTING IN LESS THAN 6 HRS OF SLEEP; LACK OF SUFFICIENT TIME TO RECUPERATE; FLYING THE BACK OF THE CLOCK ONE DAY AND THE FRONT THE NEXT IS DANGEROUS IN THE LONG RUN.