You would say "en la noche" or "por la noche" but NEVER a la noche.
The only time "a" means "at" is when expressing clock time.
For example, "The movie begins at 2:30pm" = "La película empieza a las 2 y media."
In general, "en" is a common preposition used for the following English prepositions:
- in
- on
- at
I hope this helps you! ¡Espero que esta respuesta le ayude!
Cheryl W.