Full, peaceful sleep — the ZZZ and closed eyes make it the clearest way to say 'I'm exhausted' or 'good night' without typing anything, distinct from the merely sleepy or drowsy variants. It's one of the most straightforwardly literal faces in the entire emoji set, rarely used ironically.
Keywords: bed, bedtime, face, good, goodnight, nap, night, sleep, sleeping, tired, whatever, yawn, zzz