Final answer:
The prepositional phrase 'during the evening hours' in the sentence functions as an adverbial phrase, indicating when the room is dark.
Step-by-step explanation:
The sentence 'The room is unnervingly dark during the evening hours' contains prepositional phrases acting as adverbials of time. In this case, the phrase 'during the evening hours' serves to answer when the room is dark, indicating time. Therefore, the prepositional phrase acts as an adverbial phrase, providing context to the verb 'is' regarding when the room is dark.