Final answer:
The coordinating conjunction in the sentence is 'but'.
Step-by-step explanation:
The coordinating conjunction in the sentence is but. Coordinating conjunctions are used to join two independent clauses. In this case, 'Jim Halpert wanted to stay up until midnight on New Year's Eve' is one independent clause, and 'he got too sleepy' is another independent clause. The coordinating conjunction 'but' joins these two clauses together.
Learn more about Coordinating conjunctions in English grammar