ALTDEV ALT EXCURSION IN A CFTT IN DESIGNATED MOUNTAINOUS TERRAIN.

Narrative

AFTER BEING CLRED FOR ILS 16R AT SALT LAKE CITY I INITIATED MY DSCNT EARLY OUT OF GS INTERCEPT ALT. THERE IS A STEP DOWN FIX AT 11.5 DME AND AT THAT POINT I STARTED DOWN THINKING I WAS GOING DOWN TO 6100 FT (GS INTERCEPT ALT) HOWEVER I WAS ALREADY AT 6100 FT MSL. I DSNDED DOWN TO 5800 FT BEFORE REALIZING MY MISTAKE. AS I STARTED DOWN I ASKED MY FO FOR THE GS INTERCEPT ALT BUT HE GOT BUSY SWITCHING OVER TO TWR. I STARTED LOOKING AT MY APCH CHART AND NOTICED THAT I WAS ALREADY BELOW WHERE I NEEDED TO BE. I IMMEDIATELY INITIATED A CLB BACK UP TO 6100 FT. I THINK IT IS AN EXTREME COINCIDENCE THAT NEITHER MY FIRST OR SO CAUGHT THIS MISTAKE IMMEDIATELY. I BRIEFED THE APCH DURING THE DSCNT AND IT IS REQUIRED THAT THE SO RECORD THE GS INTERCEPT ALT AND MONITOR THE APCH TO INSURE THAT IT IS FLOWN PROPERLY. I ATTRIBUTE THIS MISTAKE TO EXTREME FATIGUE. ALL 3 OF US HAD BEEN UP MOST OF THE NIGHT AND WERE EXTREMELY SLEEPY. I CONSIDER THIS MISTAKE VERY SERIOUS DUE TO THE CLOSENESS OF THE GND IN THIS CASE. I PLAN TO; IN THE FUTURE BRING MYSELF TO A MUCH HIGHER LEVEL OF ALERTNESS IN THIS PHASE OF FLT.