CONFLICT BTWN 2 ACFT OCCURRED WHEN CTLR SIC AT ZDV COMBINES POS DURING LIGHT TFC PERIOD.

Narrative

I WAS ASSIGNED THE CIC POS AT ZDV; AREA 2; WHEN THE INCIDENT OCCURRED. THE AREA WAS STAFFED WITH ONLY 12 CERTIFIED CTLRS; INCLUDING MYSELF; FOR THE EVENING SHIFT; DOWN FROM OUR NORMAL STAFFING OF 16 PEOPLE. SEVERAL PEOPLE WERE ON LEAVE OR CALLED IN SICK FOR THIS SHIFT. UPPER MGMNT WOULD NOT AUTH ANY OVERTIME DUE TO BUDGET CONSTRAINTS. THE AFTERNOON AND EVENING SHIFTS WERE QUITE BUSY. MOST CTLRS; INCLUDING OUR SUPVR; WERE ON POS FOR 90 MINS TO 2 HRS; WITH FEW BREAKS PRECEDING THE INCIDENT. WHEN THE SIC ASKED IF I WOULD OFFER HIM A BREAK AT XA17; HE HAD BEEN ON POS WELL OVER 2 HRS WITH NO BREAK. I ASSUMED SUPERVISORY CIC DUTIES AT XA17. THE AREA WAS STAFFED WITH 6 CTLRS ON POS; AND EACH OPEN SECTOR HAD FEWER THAN 5 AIRPLANES ON FREQ. SINCE TFC WAS LIGHT; THE CTLRS ON POS WERE ENGAGING IN SIDE CONVERSATIONS WHICH I FELT COULD DISTRACT THEM FROM THEIR ASSIGNED DUTIES. THIS FACTOR; ALONG WITH THE KNOWLEDGE THAT MOST CTLRS ON DURING THE SHIFT WERE FATIGUED DUE TO THE EARLIER HVY TFC PUSHES; LED TO MY DECISION TO COMBINE SECTORS AND GET MY CTLRS OUT TO REST. COMBINATION OF SECTORS IS A COMMON PRACTICE AMONG ALL SUPVRS AT THAT POINT IN AN EVENING SHIFT. I PROCEEDED TO COMBINE OUR DEN HIGH SECTOR WITH OUR DEN LOW SECTOR. THE CTLR THAT HAD BEEN WORKING THE DEN HIGH SECTOR WAS THEN MOVED TO THE NORTH PLATTE HIGH SECTOR. AFTER OBTAINING A POS RELIEF BRIEFING FOR LBF HIGH SECTOR; THE CTLR THEN ASSUMED THE OVERLYING ULTRA-HIGH ALT SECTOR. THE CLOSING OF THE ULTRA-HIGH SECTOR ONLY ADDED 2 ACFT TO THE NORTH PLATTE HIGH SECTOR AIRSPACE AND WORKLOAD. WITHIN 5 MINS OF ASSUMING THE LBF HIGH SECTOR; THE CTLR SHIPPED ACFT X WHO WAS LEVEL AT FL310 TO SECTOR 15 (DEN LOW). ACFT X WAS AN APA ARR. SECTOR 15 HAS CTL FOR DSCNT OF ARR TFC 30 MI E OF THEIR BOUNDARY. AT XA35; THE SECTOR 15 CTLR CLRED ACFT X TO FL240 PLT'S DISCRETION; FAILING TO REMEMBER ACFT X; A DEN DEP; WAS DIRECTLY UNDERNEATH AT FL290. ACFT X STARTED DOWN IMMEDIATELY AND SEPARATION WAS LOST. AT THE TIME OF THE ERROR; I WAS COLLECTING FLT PROGRESS STRIPS FROM THE CLOSED SECTORS FOR FILING. I HAD COMPLETED THE COMPUTER ENTRIES FOR SECTOR COMBINATION AND I HAD MONITORED THE AREA TFC FLOWS AT EACH SECTOR JUST AFTER COMBINATION AND I WAS SATISFIED THAT TFC WAS LIGHT AND THAT ALL SECTORS WERE UNDER CTL. I FELT NO FURTHER ACTION WAS NECESSARY. 5 MINS LATER; THE ERROR OCCURRED.