Do you have frequent urination at night time?Are you taking any medications?
Do you have painful urination? Any other symptoms?
Well, it can be due to various cause like- Chronic or recurrent urinary tract infection, Diabetes to name a few. You need to undergo certain tests like - Urine analysis, blood sugar levels, blood urea nitrogen .You should not take fluids before sleeping Also get evaluation and further treatment from your doctor.Linkhttp://www.medhelp.org/Medical-Dictionary/Terms/1/003141.htm
A normal urine culture rules out the possibilty of urinary infection. You need to get checked blood sugar levels, as high suagr level, causes frequent urination.Also one cause is -Interstitial cystitis, it is very common in feamles. take an appointment with your doctor.