AN IFR COLUMBIA 400 PILOT BEGAN A DSCNT WITHOUT ATC CLRNC WHILE DISTRACTED ON A VMC DAY AND MINDLESSLY FOLLOWING THE ACFT'S FMC.

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WHILE ON AN IFR CLRNC IN CRUISE FLT; I WAS HALFWAY TO MY DEST AND I LOADED MY VERT PROFILE IN THE FLT PLAN FUNCTION OF THE FMS. AS I NEARED THE 'TOD' (TOP OF DSCNT); I STARTED MY DSCNT TO STAY AHEAD OF THE AIRPLANE. AFTER DSNDING JUST OVER 1000 FT I REALIZED THAT I HAD FORGOTTEN TO ASK FOR A LOWER ALT FROM CTR. AT THIS POINT I CALLED ATC AND ASKED TO CANCEL IFR. THE CTLR SAID SOMETHING LIKE; 'WELCOME BACK; CONTACT CTR ON 1XX.6 AND LET THEM KNOW OF YOUR INTENTIONS.' SO NOT ONLY DID I BEGIN A DSCNT WITHOUT A CLRNC; BUT I MUST HAVE MISSED A RADIO CALL ALSO. I CONTACTED THE NEW CTLR; AND TOLD HIM 'CTR...ACFT X...CANCEL IFR.' HE ACKNOWLEDGED AND TOLD ME TO SQUAWK VFR. PART OF THE CAUSE FOR THE ABOVE INCIDENT WAS THE WX WAS SO CLR; VERY VFR. I HAD BEEN IN VFR CONDITIONS SINCE I BROKE OUT ON TOP OF A MARINE LAYER AT 2500 FT ON MY CLBOUT JUST AFTER TKOF. I HAD BEEN CLRED DIRECT TO ZZZ; AND HAD BEEN IN VFR CONDITIONS FOR OVER AN HR WITHOUT COMING NEAR ANY CLOUDS. FOR SOME REASON; I DIDN'T REMEMBER BEING ON AN IFR CLRNC; AND I JUST STARTED MY DSCNT USING THE AUTOPLT AT THE 'RECOMMENDED' TIME ACCORDING TO THE GPS. ANOTHER CAUSE MIGHT HAVE BEEN BEING TIRED FROM A LONG VFR FLT THE PREVIOUS DAY; AND NOT GETTING ENOUGH SLEEP. TO PREVENT THE SITUATION FROM RECURRING I MUST REMEMBER THAT WHENEVER I LOAD THE 'VERT PROFILE/DSCNT' INTO THE FLT PLAN FUNCTION OF THE FMS THAT I MUST ASK FOR THE CLRNC BEFORE BEGINNING THE DSCNT. I WAS TRYING TO STAY AHEAD OF THE AIRPLANE; BY PLANNING AHEAD; AND PRELOADING THE DSCNT. BUT I STILL NEED TO VERIFY THAT I ASK PERMISSION FOR A LOWER ALT WHEN THE 'TOD' IS GETTING CLOSE. IT IS ALSO IMPORTANT TO GET THE PROPER AMOUNT OF REST BEFORE FLYING; SO THAT 'STUPID' MISTAKES LIKE THIS ARE LESS LIKELY TO HAPPEN.