Parts of speech question help!?

'At night' is a prepositional phrase used as an adverb. It's no different than 'after school,' which is also a prepositional phrase also used as an adverb. For a prepositional phrase, you need a word, which is often an adverb, and an object of the preposition, a noun or a pronoun. 'After' by itself is an adverb; as part of the phrase, it becomes a preposition.