AIR TAXI CARGO IN A PA28-200R FALLS ASLEEP AND AWAKENS 30 MI BEYOND HIS DEST ARPT. HE CANCELS IFR AND RETURNS TO MAKE A LNDG.

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I WAS ON A ROUTINE CARGO FLT FROM MKE TO GBG BY WAY OF RFD VOR. THE WX WAS VMC WITH ABOUT 20 MI VISIBILITY. THE FLT AT 6000 FT STARTED UNEVENTFULLY. I WAS HANDED OFF TO QUAD CITIES APCH AND INFORMED THEM THAT I HAD GALESBURG AWOS. SHORTLY; THEY GAVE ME A DSCNT AT PLT'S DISCRETION TO 3000 FT. AS USUAL I REMAINED AT 6000 FT INTENDING TO START MY DSCNT UPON XING THE MZV 110 DEG RADIAL. ABOUT 20 MI NE OF GALESBURG AT 6000 FT WITH THE AUTOPLT ENGAGED; I FELL ASLEEP UNTIL I REACHED A POINT ABOUT 30 MI SW OF GALESBURG. UPON WAKING AND REALIZING THAT I HAD OVERSHOT MY DEST I TURNED AROUND AND HEADED BACK TO GALESBURG. I IMMEDIATELY CALLED QUAD CITIES APCH AND CANCELED MY IFR FLT PLAN. THEN I PROCEEDED TO LAND AT GALESBURG. ALTHOUGH I FELT AWAKE AND ALERT WHEN I LEFT MILWAUKEE; I FEEL THAT AN UNRESTFUL NIGHT'S SLEEP DUE TO SOME PRESSING PERSONAL MATTERS WAS A CONTRIBUTING FACTOR IN THIS EVENT. ADDITIONALLY; THE ROUTINE NATURE OF THE FLT; THE SMOOTH AIR; WARM CABIN; AND LITTLE RADIO TFC ALL HELPED TO BRING ON DROWSINESS. I BELIEVE THAT A MORE RESTFUL NIGHT'S SLEEP AND KEEPING THE CABIN COOLER BY THE USE OF VENTS AND LOWER HEATER SETTINGS WOULD HAVE PREVENTED THIS SIT.