Bedwetting is not your child's fault and there's often no obvious reason why it happens. In many cases, the problem runs in families.
Most experts believe there may be more than one underlying cause.
The bladder is a hollow, balloon-like organ located in the pelvis that's used to store urine. Once the bladder is full, urine passes out of the body through a tube called the urethra, located in the centre of the penis in boys and just under the main opening of the vagina in girls.
Some children affected by bedwetting have what's known as overactive bladder syndrome. This is where the muscles that control the bladder go into spasm, leading to the involuntary passing of urine.
Excessive urine production
Urine is produced by the kidneys. The kidneys remove waste products from the blood. These are mixed with water to produce urine, which then flows into the bladder.
The more fluid your child drinks, the more urine their kidneys produce. Therefore, if your child drinks lots of fluids during the evening, it could result in them wetting the bed during the night, particularly if they have a small bladder capacity. Drinks that contain caffeine, such as cola, tea and coffee can also stimulate an increase in the production of urine.
In some cases of bedwetting, it may be that the child's body doesn't produce enough of a hormone that regulates urine production, called vasopressin. This means their kidneys produce too much urine for their bladder to cope with.
Difficulties waking up during the night
Once the amount of urine in the bladder reaches a certain amount, the bladder should send signals to the brain.
The signals should convey the feeling of needing to go to the toilet, which would cause most people to wake up. However, some younger children are particularly deep sleepers, and their brain doesn't respond to the signals being sent from their bladder, so they don't wake up.
Alternatively, in some children the nerves attached to the bladder may not yet be fully developed, so they don't generate a strong enough signal to send to the brain.
Sometimes, a child may wake up during the night with a full bladder but not go to the toilet. This may be due to childhood fears, such as being scared of the dark.
Underlying health conditions
Bedwetting can also be caused by an underlying health condition, such as:
constipation - if a child's bowels become blocked with hard stools (faeces), it can put pressure on the bladder and lead to bedwetting
type 1 diabetes - a lifelong condition that causes a person's blood sugar level to become too high and can result in the excessive production of urine
urinary tract infections (UTIs) - a UTI is an infection of the urinary tract which consists of the urethra, the bladder, the kidneys and the ureters (the tubes that connect the kidneys to the bladder)
abnormalities with the urinary tract - such as bladder stones
damage to the nerves that control the bladder - this could be due to an accident or a condition such as spina bifida
In some cases, bedwetting can be a sign that your child is upset or worried. Starting a new school, being bullied or the arrival of a new baby in the family can all be very stressful for a young child.
If your child has started wetting the bed after previously being dry for a period of six months or more (known as secondary nocturnal enuresis), emotional problems such as stress and anxiety may be responsible.