The hyphen is just the standard way of writing it (vd. http://bertilow.com/pmeg/gramatiko/e-vortecaj_vortetoj/lokaj/chi.html#i-h6f)
I don't think anybody would understand it as meaning anything else if you left it out, though. "ĉi nokte" (space, no hyphen) looks less weird to my eye than "ĉinokte" (no space) for whatever it's worth.
As I understand it, we need to use the adverb to describe how (when) she's going to sleep, not what she's going to sleep...because that doesn't make sense...
In English, "tonight" is an adverb as well as a noun, so in that same sentence, we'd use it as an adverb, not a noun.
Hope that helps.