While you were sleeping, I watched a movie. Is this a Simple, compound, or complex sentence

It's a complex sentence because it has an independent clause (I watched a movie) and a dependent clause (while you were sleeping)

While you were sleeping, I watched a movie is a complex sentence.

Explanation:

Complex sentences are those that contain an independent clause and a dependent clause. Independent clauses, like I watched a movie, stand alone as a sentence. Dependent clauses, although they include a subject and a verb, cannot stand alone and are usually linked to the independent clauses by using a coordinating conjunction.