There are several synonyms to express the meaning of "sleep before midnight." One of the first and most common words is "early to bed," which directly implies getting into bed before the clock strikes twelve. Another phrase that also carries the same meaning is "hit the hay early," which is a more informal way of saying the same thing. You can also use "retire early," "go to bed at a reasonable time," or "catch some Z's before midnight." All of these synonyms emphasize the importance of getting sufficient rest and suggest that going to bed early is an essential aspect of maintaining good sleep hygiene.