A child obtains bladder control between 18-24 months. Once the child has bladder control, the central nervous system produces a hormone that reduces urine production during REM sleep. The body controls urine output at night; there is nothing a parent needs to do to help once the child is asleep. Many parents assume their child won't make it through the night dry because they have a full diaper in the morning. A full diaper in the morning is a sign the child is making it through the night dry and emptying a full bladder when they wake up out of REM mode. When a parent places a diaper, pull-up, or underwear on the child, when the child wakes up in the morning, the first thing they do is pee in it. If a child has anything wrapped around his or her waist when they wake up sleepy, they don't know it is not a pull-up, they just feel the regular snug feeling, and out of habit, they pee in it. To set your child up for success, put your child to bed naked from the waist down. Night training is breaking a habit. Putting your child to bed without any underwear, PJ bottoms, pull-ups, or diapers, will help your child break the habit of peeing the bed. A change in routine/feeling will make your child more aware of the urge to pee and help them decide to get out of bed and use the toilet.
Kids are awake when they pee the bed; they do not pee in their sleep. Just like adults, our body wakes up when our bladders are full, even in the deepest periods of sleep. Think of it this way; many adults will heavily sedate themselves with sleeping pills, alcohol, or prescription pills and not pee the bed. Children are no different; when they wake up in the morning, the bladder fills up, and the child feels the urge to pee.
Many things can wake a child up out of REM sleep; a coffee pot, mom or dad's alarm clock, a car driving by, etc. Any of these things may wake a child up out of REM sleep. It is common for a child to wake up around 5:00 AM, pee the bed, and fall back asleep. It is helpful to wake a child up 10-20 minutes before they usually wake up to not allow them to pee the bed. Helping your child break this habit is key to success.
It is essential to talk to your child, so they know you expect them to stop peeing in the bed and use the toilet. Before you put your child to bed, make sure they have emptied their bladder. Measuring the child's urine output will ensure they have fully emptied their bladder. A child's bladder holds their age plus 2 in ounces. Use a specimen catcher to measure this amount before bed. Explain to your child why they are not wearing a diaper, pull-up, PJ bottoms, or underwear to bed. Tell them that they will need to get out of bed and use the toilet when they feel the urge to pee. Role-play with your child so they understand. Tuck your child in bed and have them show you what they will do if they need to pee. Have them get out of bed, walk to the toilet, and tuck themselves back in bed. I recommend role-playing 5 times. Doing this routine before bed will be the last conversation your child hears before falling asleep, making it easy to remember when they wake up in the morning.
Stick to this routine for a minimum of one week to break the habit of peeing in the bed. You may get a few nights with wet sheets while your child learns to break this habit and respond to the feeling of a full bladder. One last thing, when the child does not have a bowel movement during the day, almost all children pee the bed at night. It's not a lot of pee, more like dribble due to the stool pushing against the bladder.