Those in sleep medicine fellowships might disagree with you. So would the general membership of the American Board of Sleep Medicine. Unless I'm misunderstanding your unusual comment?
You find factual comments unusual? Why?
It doesn't matter who disagrees with me but Sleep Medicine is not an official ABMS specialty. That's a fact. You can't disagree with a fact.
See here: http://www.certificationmatters.org/abms-member-boards.aspx
Additionally, you don't even have to be an MD to become a ABSM diplomate.
So let's try again: Sleep medicine is not a medical specialty. It might be an area of practice or a field of interest but it is not a recognized medical specialty. Sleep medicine is no more of a specialty than Naturopathic Medicine or Anti-Aging medicine. Anyone can start a Board, give exams, and start collecting fees in exchange for a certificate.
Finally, if you don't believe me ask the ACGME or HHS. The Boards they recognize are the ABMS members.