Wide awake

Why do we say "wide awake?" What is so wide about being awake? I guess "very awake" sounds strange. But I can't think of any other instance of using wide in this way. We don't say wide aware, although I think the W sounds in wide awake are euphonious (mellifluous).

I am only thinly awake right now. Barely awake? Somewhat alert?
What's the opposite of Awake? Asleep or unconscious. But there are many degrees between Awake and Asleep.

OK when I count to 5, You will find yourself back in the room, wide awake and feeling wonderful in every way.

One...
Two. ..
Three...
Four...
FIVE!