Preheat oven to 350 degrees. Arrange sweet potatoes in a 9-by-13-inch baking dish. Add sugar and bourbon and toss to combine; season with salt and pepper. Bake until sweet potatoes are tender and glazed, 1 to 1 1/2 hours, tossing every 30 minutes. Stir in butter before serving.
Place the sweet potatoes in the oven after the turkey has roasted for an hour.