In English, we usually say “Goodnight” before going to bed. But in Spanish, the literal translation of goodnight, buenas noches, is used as a greeting. The English equivalent would be “Good Evening”. Buenas noches is never used to say goodbye before going to bed.
To say goodbye to somebody at night, you can use any normal parting phrase, but hasta mañana is common if you know you will see the person the next day. To wish somebody a good night’s rest, it is common to say descanses, which is short for que descanses, or que descanses bien. Here the verb descansar (to rest) is in the subjunctive tense and the first clause is practically omitted. The first clause would be Espero que or Ojalá que. So when you hear descanses, it is shortened version of “I hope you rest (sleep) well”.
