Some of the causes of bed-wetting include:
 - Genetic factors (it tends to run in families) 
 - Difficulties waking up from sleep 
 - Slower than normal development of the central nervous system--this  reduces the child's ability to stop the bladder from emptying at night 
 - Hormonal factors (not enough antidiuretic hormone--this hormone reduces the amount of urine made by the kidneys) 
 - Urinary tract infections 
 - Abnormalities in the urethral valves in boys or in the ureter in girls or boys 
 - Abnormalities in the spinal cord
 - Inability to hold urine for a long time because of small bladder

Enuresis is the medical term for bed-wetting during sleep. Bed-wetting is fairly common. About 5 million to 7 million children wet the bed. It's more common among boys than girls.

Children achieve bladder control at different ages. By the age of 5 years, most children no longer urinate in their sleep. Bed-wetting up to the age of 5 is not unusual, even though it may be frustrating to parents.