AN AIR AMBULANCE FLT DSNDING TO 16000 FT AND ANTICIPATING A VISUAL APCH; CONTINUES DSCNT THROUGH 16000 FT. THE CTLR QUESTIONS THE PLT ABOUT HIS ALT AT 14000 FT. THE REMAINDER OF THE APCH AND LNDG WAS UNEVENTFUL.

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EARLY MORNING (MIDDLE OF THE NIGHT) AIR AMB FLT; SINGLE PLT. WX VERY GOOD; VMC ENTIRE TRIP. SHORT (35 MIN) LEG EACH WAY TO RETRIEVE PATIENT AND BRING BACK TO ZZZ. ON RETURN LEG TO ZZZ; CTR CLRED ME TO DSND AT DISCRETION FROM FL210 TO 16000 FT; APPROX 80 NM S OF ARPT ZZZ. ABOUT 50 NM S I BEGAN DSCNT AND ADVISED CTR. ABOUT 40 NM S; CTR ASKED ME TO CONTACT APCH ON 118.3 (EVERYTHING ON TWR FREQ; THIS TIME OF DAY XA00 LCL). I WAS LISTENING TO ATIS AT THE SAME TIME. I ACKNOWLEDGED CTR AND CONTINUED TO LISTEN TO ATIS WHICH WAS ADVERTISING VISUALS TO RWY 03; WHICH I WAS EXPECTING. I RESET MY ALT ALERTER TO INDICATE 'CLRED FOR VISUAL APCH' ERRONEOUSLY. I CHKED IN WITH TWR AND APCH. TOLD HIM I WAS DSNDING FOR RWY 3. WHEN HE ASKED MY ASSIGNED ALT; I KNEW SOMETHING WAS WRONG. I WAS NOW ABOUT 14000 FT AND LAST CLRNC WAS 16000 FT. UNEVENTFUL APCH AND LNDG FOLLOWED. NO CONFLICTS OR LOSS OF SEPARATION. PRIMARY FACTORS: TIME OF DAY: VERY TIRED; FAR BELOW 100 PERCENT MENTAL ALERTNESS. IT EVEN TOOK ME A WHILE TO FIGURE OUT WHAT I HAD DONE. OTHERS: EXPECTATIONS AND COMPLACENCY. EXCELLENT WX; VERY FAMILIAR WITH ALL ASPECTS OF THIS FLT. ARPT (AND EXPECTED RWY) IN SIGHT FOR A LONG TIME. I MENTALLY REPEATED 'VISUAL RWY 3' WHEN I HEARD IT ON ATIS AND PROCEEDED TO DO THAT; ALT WISE. THERE WAS NOT A COURSE DEV AS THE COURSE TO THE VOR SET ME UP FOR RWY 3 (ALSO A CONTRIBUTOR). CORRECTIVE: EXERCISE EXTREME VIGILANCE WHEN OPERATING ACFT AT TIMES YOU SHOULD BE AND NORMALLY WOULD BE SLEEPING. BE ALERT FOR 'EXPECTATION ERRORS' WHEN EVERYTHING IS FAMILIAR.