Adverb sentences have the function of an adverb, and in them we have some subordinate conjunctions to connect the dependent sentence (adverbial sentence) to the independent sentence. In fact, the conjunction connects the adverbial prayer with the word contained in the independent prayer that the adverbial prayer will modify.
The adverb clause in this sentence is since she had a test tomorrow, and it shows the reason why Colleen went to bed early. The subordinating conjunction is since. The words modified by this adverbial clause are went to bed early.
