In addition to the arousal disorder which affects the brain response to bladder signals, some bedwetters have a small bladder.

A small bladder cannot hold as much urine as a normal sized bladder. Nor can a bladder which has a tendency to go into muscular spasms (irritable bladder).

A child with a small or irritable bladder will also need to urinate frequently during the day, often with urgency. A medical assessment should be made if the child suffers from day-wetting. Sometimes the bladder spasms only happen during sleep.

Once the bedwetter has trained their brain to wake to bladder signals during sleep with a bedwetting alarm, they can stay dry - they just need to go to the toilet more often than someone with a normal sized bladder.

Once night control is established, bladder size will often increase as a consequence.