As you drink water during the day, your body turns extra liquid it doesn't need into pee. Your bladder stores the urine and eventually alerts you when it's time to take a trip to the toilet.
But what about at night? How does your body know not to pee while you're asleep?
Just because you're snoozing doesn't mean your body is totally offline - continuous processes like breathing, digestion and, yes, making pee, still happen while you're asleep. Your bladder and your brain work together to know what to do with that big glass of water you drank before bed.
Using the bathroom every day is routine for many people, so it's something you might not pay much attention to. But as a pediatric urologist, understanding how the brain and bladder work together - and sometimes miscommunicate - is an important part of my job.
The bladder and the brain
The bladder has two main jobs: to safely store urine and to empty it out. While it seems simple, these two tasks take a lot of complex coordination of muscles and nerves - that's the brain's job.
For babies and young kids, the bladder has reflexes, meaning it automatically knows when to squeeze the muscles to empty the urine. Since babies can't control this consciously, they typically wear diapers. But as kids grow, the bladder muscles and nerves also grow, which gives a youngster more control over their bladder.
During toilet training, which usually happens by the age of 3 or 4 in the U.S., kids learn how to use the toilet voluntarily. This means that they can feel when the bladder is getting full and their brain can receive and understand that signal. The brain can then tell the bladder to "hold it" until they've made it to the toilet and it's safe to pee.
What happens in sleep mode?
Most children first learn how to use the toilet during the day. Using the bathroom overnight can be more difficult because the sleeping brain doesn't receive signals in the same way as when awake.
While awake, if there's a loud noise or a bright light, the body senses it and reacts. But during sleep, the body may not hear that noise or see that light because the brain is in sleep mode. Imagine sleeping through an overnight thunderstorm that you didn't realize happened until you hear people talking about it in the morning. Your brain didn't process the loud noises because it was focusing on sleep.
The same thing can happen with bladder signals. The bladder fills with urine 24 hours a day, even while you're snoozing, and it sends signals to the brain when it's full. In order to help you get enough sleep, your brain will tell your bladder to hold it until morning.
Sometimes, if you really need to go, your brain will tell your body to wake up so you can go empty your full bladder. While it's normal to wake up to pee sometimes - especially if you drank a big cup of hot chocolate right before bed - most older kids can usually sleep through the night without needing to use the toilet.
When the brain and bladder are working together well, your bladder gradually fills up overnight and hangs on til morning when you stumble into the bathroom to empty it.
But there are many ways the communication between the brain and the bladder can break down. For one, the brain may not get the bladder's message that it's time to go. Even if the brain gets the message, it may not be able to tell the bladder to hold on. Or, when the bladder can't wait, the brain might not tell your body to wake up. If the signals and messages aren't sent, or are received incorrectly, the bladder will go into reflex mode - it squeezes to empty itself of pee, even though you're fast asleep in bed.
Wetting the bed at night, which doctors call nocturnal enuresis, is more common than you might think. About 15% of kids between ages 5 and 7 wet the bed sometimes. Even some teenagers experience it. It's more common in boys, and often there's a family history, meaning parents or relatives may have dealt with nighttime accidents too.
There are a few reasons why nighttime wetting happens. Since kids' brains are growing and developing, nighttime communication between the brain and bladder can take longer.
Some bodies make more pee at night, making it more likely the bladder will get full during sleep. Some people have smaller bladders that fill up fast. Sometimes having difficulties with sleep or being a deep sleeper can make it harder to wake up at night if you really need to pee.
Most kids who wet the bed at night outgrow it as their brains and bodies continue to develop. At that point, they can sleep through the night without needing to pee, or their bodies are able to wake up at night to use the bathroom when they need to.
If wetting the bed is an issue, there are some things that can help, like drinking less liquid in the evening or using the bathroom right before you go to bed. These precautions make it less likely that the bladder will be too full during sleep. There are also bedwetting alarms that can help train the body to wake up when the bladder needs to be emptied. If there are concerns about nighttime accidents, or if accidents start happening in older children, I recommend consulting a doctor.
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Author: Jennifer Ahn - Assistant Professor of Urology, School of Medicine, University of Washington