It is the most common form of bedwetting. Bedwetting is considered as a disorder once the child is old enough not to urinate or “wet the bed”, but continues to do so. It affects children 5 years or over who have never achieved dryness at night for a continuous period of 6 months.
Several definitions have been used for nocturnal polyuria. It may occur when 24-hr urine production is within normal limits and nocturnal polyuria index is greater than 0.33. Others define it as when the nocturnal urine volume is greater than 6.4 ml/kg, nocturnal urine output is greater than 0.9 ml/min (54 ml/hr) or higher and greater than 90 ml/hr. This may be caused by excessive fluid intake in the evening, diuretics, the disruption of normal vasopressin secretion and the like.