One of the most common questions I get from parents is some version of this: my child can hold it during the day for hours, so why does she wet the bed at night? It is a fair question, and the answer turns on a distinction that most people never learn. Daytime restraint and nighttime restraint are two different jobs, run by two different parts of the brain.
The same plumbing, two different brains
The urinary system is identical in both states. The kidneys produce urine, the ureters carry it to the bladder, the bladder fills, and at some point the sphincter has to do its job. What changes between day and night is which part of the brain is in charge of telling the sphincter what to do.
During the day, while the child is awake, the cognitive brain handles it. The child feels the bladder fill, decides whether now is a good time to use the toilet, and acts accordingly. This is voluntary, conscious, and trained over the first few years of life.
At night, while the child is asleep, the cognitive brain is offline. There is no conscious decision-making happening. The job falls to a subconscious reflex mechanism whose only task is to keep the sphincter closed until the bladder pressure crosses a threshold the brain notices.
Why one can work without the other
A child can have a perfectly competent daytime system and a nighttime system that has not yet learned its job. This is why "she has no problem during the day" is not evidence that the bedwetting will resolve on its own. The daytime mechanism is mature. The nighttime mechanism is the one that needs training.
A quick tour of the bladder
The bladder itself is a muscular bag, controlled by smooth muscle that operates without conscious input. It has two sphincters: an inner one that responds to the involuntary system, and an outer one in the urethra controlled by voluntary muscle. The outer sphincter is the one the cognitive brain uses during the day. The inner sphincter is the one the sleeping reflex relies on at night.
Why this matters for treatment
Daytime exercises that strengthen voluntary control, useful as they are, do not directly train the involuntary nighttime mechanism. What trains that mechanism is repeated pairing of bladder signal with response, which is exactly what an alarm-based programme does during sleep. The brain learns through experience, even while the conscious mind is unavailable. Read more about the science of bedwetting.