Accidents are probably caused by many factors, not just one. Many children with nighttime urine accidents have an overactive bladder and a small functional bladder capacity. This means that a bladder may have many muscle spasms that cause an urge to empty the bladder often. The bladder is made of muscle and can grow and shrink as urine enters or leaves. The bladder in most cases is a normal size physically. However, it is not able to hold a normal amount of urine because of muscle spasms.
Other possible causes of nighttime urine accidents include hormonal factors, dietary factors, the effects of certain medications, timing of daytime fluid intake and emotional factors. Often, nighttime accidents are found to run in families.
Some children who have nighttime accidents will also have daytime urine accidents. Causes for this may include urinary tract infections, abnormalities of the urinary tract, problems with bathroom habits, or problems with the nerve control of the bladder.