I, for one, side with pablo's view that there is a more sinister undercurrent here. A 4-year-old who regularly (I presume) has headaches in the morning, then there is something wrong, right? Then you have to be hyperactive in a pathological sense.
Are we completely sure of the lyrics here? It couldn't be anything else than "I get headaches in the morning", could it?
It could also be, that Bowie was a hyperactive child who was plagued by headache as a little child himself and, when he wrote this lyric, didn't reflect very deep but just included it as one of the typical features of four-year-old-hood that he could think of. I don't know.
Another thing... Bowie did not want the song to be reissued on the Deram Anthology. Is that just because it's too silly ... or - in line with pablo's theory - too personal?