Causes

Difficult to rouse from sleep
Children who wet the bed have great difficulty rousing from sleep when their bladder is full. The bladder and the brain do not communicate properly so that when the child is sleeping, the brain doesn't get the message that the bladder is full and needs to be emptied.

Overactive or "twitchy" bladder
The bladder muscle becomes twitchy and can only hold small amounts of urine. The twitchy bladder may spontaneously contract during sleep which can result in wetting.

Overproduction of urine during the night
Children who wet the bed may have a low level of a naturally occurring hormone which the brain normally produces during the night to reduce the amount of urine produced and allow an uninterrupted night's sleep. If the child has low levels of this hormone at night, they may produce more urine than their bladder can hold and, if they do not wake up, they wet the bed.