Have you eaten something before going to bed, such as watermelon? Also, are you diabetic? Are you currently on any medications? 8 to 16 ounces is not a small amount.
Does this symptom persist? If not, it is most likely to be caused by something you have eaten or drunk before going to bed. If this persists, it would indicate a problem, such as diabetes insipidus, which needs to be evaluated by a doctor.