PLT OF AN SMA SEL DSNDED BELOW THE 12000 FT ASSIGNED CRUISING ALT CAUSING ATC TO INTERVENE TO RETURN RPTR TO ASSIGNED ALT. CAUSE FOUND TO BE AN APPARENT LACK OF OXYGEN TO THE PLT CAUSING HYPOXIA.

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PERFECT FLYING CONDITIONS -- 4 PLACE; SINGLE ENG; RETRACTABLE GEAR ACFT WITH HEADING HOLD AUTOPLT AND NO ALT HOLD. LONG FLT; NEAR MAX RANGE SO CRUISING AT 12000 FT FOR MAX RANGE AND EFFICIENCY; OXYGEN ON BOARD BUT NOT IN USE. APPROX 4 HRS INTO FLT; CTLR ASKED MY ALT AND I COULDN'T BELIEVE MY EYES. IT LOOKED LIKE 11300 FT BUT I THOUGHT I MUST BE WRONG. I MUST BE READING THE WRONG NEEDLES SO I ASKED HIM WHAT HE SHOWED FOR AN ALT AND HE SAID 11300 FT. I REALIZED THEN THAT I HAD HYPOXIA. I WENT ON OXYGEN AND DOWN TO 6000 FT. I FELT FINE IN ALL RESPECTS BUT SOMEHOW I HAD LOST 700 FT WITHOUT REALIZING IT. I'M A NON-SMOKER AND WALK 20 MI PER WK; SKI IN THE MOUNTAINS IN THE WINTER AND I AM FAMILIAR WITH HYPOXIA FROM THE MIL. I WAS SURE THAT I COULD RECOGNIZE LACK OF OXYGEN BUT I WAS WRONG. IT WAS VERY INSIDIOUS AND I HAD NO INDICATION THAT ANYTHING WAS AMISS. I DON'T THINK IT WAS THE KIND OF THING THAT WOULD CAUSE AN ACCIDENT BY ITSELF; BUT IT COULD CAUSE POOR DECISION MAKING WHICH COULD CAUSE AN ACCIDENT. I THINK FOR EXAMPLE OF ENG FAILURE AND MY ABILITY TO REMEMBER WHAT TO CHK; AND THEN MY SKILLS AT DEAD STICKING INTO A FIELD OR ROAD. I HOPE THAT I LEARNED MY LESSON AND I HOPE YOU WILL SPREAD THE WORD.