"Sweet dreams!" (5,4)
'sweet dreams' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for night that I've seen before include "Time for bed" , "A Hard Day's (the Beatles)" , "- School, - owl" , "Opposite of day" , "8d [DARKNESS]" .)
(Another definition for sleep well that I've seen is " Good night wish".)