THE CAPT OF A B757-200 THINKS HE MAY HAVE EXPERIENCED A MICRO-SLEEP EVENT; DURING AN APCH WITH ZDC; CAUSING HIM TO CROSS LIZIO ABOVE HIS ASSIGNED ALT.

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I WAS GIVEN A DSCNT TO CROSS LIZIO AT FL250. AS THERE WAS WX UNDERNEATH US; I ELECTED TO WAIT UNTIL THE LAST MOMENT TO DSND. THE FMC WAS PROGRAMMED AND; AS I APCHED THE DSCNT POINT; MY FINGER ON THE FLT LEVEL CHANGE BUTTON; I NOTICED A SKIP ON THE SCREEN AND WE WERE PAST THE DSCNT POINT. I SELECTED FLT LEVEL CHANGE AND CLOSED THE THROTTLES. THE DSCNT POINT AT FL250 NOW SHOWED BEYOND LIZIO. THE SPD BRAKE WAS DEPLOYED HALF-WAY AND THE DSCNT ARC SHOWED AT LIZIO. AFTER A MIN; THE DSCNT ARC WENT BEYOND LIZIO AND MORE SPD BRAKE WAS DEPLOYED; BRINGING THE ARC BACK TO LIZIO. JUST BEFORE LIZIO; I WAS GIVEN A DSCNT TO 15000 FT; SO THE SPD BRAKE WAS LEFT IN THE DEPLOYED POS AND I WAS DSNDING AT MORE THAN 6000 FPM. AS THE MILEAGE AT LIZIO WENT TO ZERO; I WAS OUT OF FL265 AND; SEVERAL SECONDS LATER; OUT OF FL250; RESULTING IN BEING TOO HIGH AT LIZIO. NOTHING WAS SAID; AND THE REST OF THE FLT CONTINUED NORMALLY. MY CONCLUSION IS THAT I DID NOT HAVE A SCREEN SHIFT; BUT THAT I HAD FIXATED ON THE SCREEN; EYES WIDE OPEN; AND HAD A SLEEP EVENT. THIS ALL HAPPENED IN A SHORT PERIOD OF TIME; WHILE THE FO WAS GETTING THE ATIS AND CALLING COMPANY.