In the brain area, what function controls the sleep cycle?

(a) pons
(b) midbrain
(c) mdulla oblongata
(d) hypothalamus

When I looked in my medical terminology book,it had told me something else and these did not seem to be th answer. According to the book it was The Reticular formation controls the sleep. HELP! Am I missing something?

According to this site, it's the hypothalmus.

http://news.harvard.edu/gazette/1996/03.14/SwitchToTurnOnS.html