WDB CREW; DSNDING INTO IAD; OVERSHOT AN ASSIGNED ALT.

Narrative

I WAS HAND FLYING DURING DSCNT INTO IAD ON THE JASEN ARR. NO TFC; GREAT VISIBILITY; NO CLOUDS. WE WERE CLRED DIRECT GILBY (ON ARR) TO CROSS AT 6000 FT; WHICH WAS SET AND VERIFIED IN ALT WINDOW. THIS CLRNC WAS GIVEN FURTHER OUT THAN USUAL AND I STARTED A SHALLOW DSCNT. HOWEVER; THE PUBLISHED 'EXPECT ALT' OF 5000 FT AT GILBY WAS LEFT IN THE FMC; SO 5000 FT APPEARED ON THE NAV DISPLAY UNDERNEATH GILBY. AS I WAS FINESSING THE SHALLOW; PWR-ON DSCNT TO CROSS AT 6000 FT; I WAS FOCUSING ON THE GREEN ARC ON THE NAV DISPLAY AND LOOKING AT GILBY 5000 FT ON THE NAV DISPLAY. AT SOME POINT; I STARTED THINKING 5000 FT WAS ASSIGNED XING ALT. WITHIN A FEW MI OF GILBY; I WENT THROUGH 6000 FT. SOON THEREAFTER; THE CAPT CALLED OUT 'ALT.' THE ALTDEV ALERT SOUNDED AND I BEGAN SMOOTHLY CORRECTING. ATC ASKED OUR ALT AS WE BOTTOMED OUT ABOUT 5500 FT AND QUICKLY GOT BACK TO 6000 FT. FATIGUE MAY HAVE BEEN A CONTRIBUTING FACTOR. I GOT UP AT XA30 FOR AN XB25 SHOW. ALTHOUGH I HAD ENOUGH SLEEP; JUST GETTING UP THAT EARLY LEAVES ME A LITTLE OUT OF SORTS. I DID FEEL TIRED DURING THE FLT; AND DURING THE LONG SHALLOW DSCNT; I THINK I BECAME CHANNELIZED ON THE NAV DISPLAY.