Nocturia, defined as waking to void at least once per night between periods of sleep, is a common complaint and shows an age-dependent increase in both prevalence and severity (number of nocturnal voids). It is the most bothersome symptom of benign prostatic hypertrophy and has been linked to an age-dependent loss in circadian release of endogenous nocturnal vasopressin and consequent over production of urine at night (nocturnal polyuria).