Should My Child Be Nighttime Potty Trained by Age 3.5?

Does nighttime potty training after 3.5 years really negatively affect muscle development, as mentioned in the Oh Crap! method? Everyone I know has used night diapers until 5. Waking a 3-year-old to pee overnight is exhausting.
—Jessie

There is a clear consensus in the medical and pediatrics literature that the ability to stay dry at night is a neurological developmental milestone. As the brain matures and the bladder gets bigger, kids will start waking up at night to pee. Most parents find that, at some point, you get enough nights of dryness that you’re ready to take the underwear plunge (get a plastic mattress protector first).

There are some simple things that can perhaps slightly speed this up. Pee before bed. Pee as soon as they get up. Make sure they know how to get to the bathroom in the night if they want to.

About 15% of children at age 5 are still peeing in a diaper or their bed sometimes at night; this declines rapidly after that. By the age of 7, if your child is still not dry at night, there may be reason to intervene — for example, by waking them up at night and limiting fluids. Bed-wetting at older ages is more common among boys than girls, and among those with a family history.

With that background: to your question. It is possible to encourage dryness in a younger child, but quite hard. Since their brain isn’t going to wake them up to pee (see point on neurological development above), you need to do it yourself and hit the right time. This is exhausting for you, and also may interfere with their sleep. The claim I think you’re referring to is the idea that the bladder muscles develop around this age, and if children are not training their bladder at night, the muscles will atrophy.

There is … just no evidence for this. And given the enormously large share of people who were not nighttime trained by 3.5, an epidemic of atrophied bladders would surely be noticed. Buy some pull-ups and get some sleep.