The correct conjunction to complete the sentence is "so." This is because "so" is used to indicate a logical consequence or result. In this sentence, being tired is the reason for going to bed early.

The conjunction "but" is not correct because it is used to indicate a contrast or opposition between two ideas. "Yet" is not correct because it is used to indicate that an action or event has not happened, although it was expected or planned. The conjunction "or" is not correct because it is used to indicate a choice or alternative between two or more options.