Getting up during the night in order to pass urine (nocturia) is a real nuisance, but one that is endured by a very large percentage of the adult population. Women are five times as likely to need to do this than men, and the problem steadily worsens with age.
Women who have had children are more affected, as the structures in the pelvis that control the bladder are stretched during childbirth and never quite return to their former strength. Women also often find that the problem is worse at different times of the month, usually just before a menstrual period, and worse again after menopause as the lack of hormones also reduces tissue strength in the pelvis.
The kidney does not produce urine at a constant rate, but reduces its urine output at night while increasing it during the day. This cycle becomes less pronounced with age as the controlling hormone from the pituitary gland under the brain fails to be produced in sufficient quantities.
There are a number of medical conditions (e.g. cystitis, pyelonephritis, diabetes) which can increase urine production or irritate the bladder to cause nocturia.