A C208 DEPARTED FAT IN IMC AND ENGAGED AUTOPLT AT CRUISE ALT. PLT FAILED TO MONITOR INSTS. WHILE DOING PAPERWORK THE ACFT CLBED ABOVE ASSIGNED ALT. AFTER REGAINING ASSIGNED ALT PLT RE-ENGAGED AUTOPLT AND DSNDED BELOW ASSIGNED ALT WHILE REACHING FOR ASRS FORM.

Narrative

AFTER A LONG WK OF SINGLE PLT IFR AND LATE NIGHTS GOING TO COLLEGE; MY DAY WAS ALMOST OVER. AFTER WAITING FOR WX MINIMUMS TO COME UP; I DEPARTED FAT FOR VIS. BEING VERY TIRED; I SET THE AUTOPLT FOR THE 15 MIN FLT AND DECIDED TO DO SOME PAPERWORK. AFTER ABOUT 3 MINS WITH MY HEAD DOWN. I WAS CALLED BY ATC AND QUESTIONED ABOUT MY ALT AND NOTICED AT THE SAME TIME THAT I HAD CLBED TO 3700 FT MSL (I WAS ASSIGNED 3000 FT MSL). AFTER CORRECTING THE ALT GAIN BACK DOWN TO 3000 FT MSL AND PRESSING THE ALT BUTTON ON THE AUTOPLT; I PUT MY HEAD DOWN AGAIN TO LOOK FOR THIS NASA RPT FORM. WHEN I LOOKED BACK UP 30 SECONDS LATER; I HAD DSNDED TO 2600 FT MSL. THIS TIME THE CTLR DID NOT SAY ANYTHING; BUT LATER; WHEN I WAS S OF VIS ARPT; RADAR WAS LOST (COMMON FOR THIS AREA). THE CTLR ASKED WHAT MY ALT WAS. '3000 FT;' I SAID. THEN HE TURNED ME OVER TO ADVISORIES AND TO CALL BACK WITH MY CANCELLATION OR MISSED APCH. I COULD HAVE PREVENTED THIS BY GETTING MORE REST THE NIGHT BEFORE AND SAVING PAPERWORK FOR THE GND.