I only slept maybe 4 hours, but it wasn't a problem at all. I did 3 or 4 practice tests on 2 hours sleep, so I wasn't worried about it.
What did annoy me though was the proctor-lady, who was just noisier than any person has a right to be. Reading the newspaper, eating cellophane-wrapped peppermints, walking around.
And then, during the verbal section she made two phone calls. She decided it would be less distracting if she opened the door to the room, and stuck her head out, while talking...as if we couldn't hear her that way.
Consequently, I did worse on my verbal than I had on any of the practice tests, and where I usually finished with 20 or so minutes to spare on the practices, I had less than 5 minutes left over on the actual MCAT.
Thus ends my tale of woe.
I don't think its my MCAT score, though, that's keeping the schools from accepting me. So, I don't think it was that big a deal.