What Causes Frequent Urination At Night?

I wake up about 2 or 3 times a night to urinate, I have had this going on for 14 or more years and that's the only symptom. Its not diabetes my blood sugar is low, the test show that I do not have an infection in my prostate, I thought it might be diabetes Insipidus but the doctor said my ADH levels were normal but with these results my reg. doctor suggested I take an enlarged prostate pill I said no, so I am going to see a Urologist on Feb. 27 2014 and could my ADH levels be in the lower normal range if such a thing exist or is there something else going on that the doctors have not found. I do not believe its an enlarged prostate oh yes I did hit my head in the 1990s falling off my bike but the doctor said no to that the hitting of the she said was not a hard enough hit. please help me.