"Li laboras dum la nokto."
Translation:He works at night.
"En" establishes a deadline or makes the clause specific to a certain time/situation.
en vetero pli bona - in better weather en Januario - in (before the end of) January en apenaŭ dek sekunoj - in no more than ten seconds
So, "en nokto" is slightly different from "dum nokto" - it implies that he doesn't work during the day.