When I mentioned bladder muscle exercises in an e-mail exchange with a teenager a few years ago, he responded:
"A couple of months ago the doctor gave me some instructions on exercises to help strengthen some muscles, and I think it helped some. I used to wet every night, now I only wet 3 or 4 times a week (well most of the time anyway). Sometime I wake up while it is happening."
There are two bladder muscle exercises I am most familiar with. Any person who has experienced reflux or kidney/bladder dysfunction other than sleepwetting or small bladder capacity should probably seek an evaluation and advice from a urologist or continence care clinic before trying either exercise.
Exercise Two -- Bladder Stretch
(This is called stretchies or hold it by some kids.)
Once a day, and ONLY once, but NOT between dinner and bedtime, drink extra liquids, note when the bladder feels full, and then see how long urination can be delayed. At other times, empty the bladder soon after full and make sure it gets completely empty. Once a week, measure the output twice, once at 'full' and then later at 'stretched' and record bladder volume. Use a big glass measuring cup from the kitchen. Just handle it carefully and wash it well when you are done.
How much can you expect a bladder to hold when a person is awake? Typically, kids age five through puberty notice that they have a full bladder when it holds about the same as their age in ounces. Most can delay voiding long enough to hold one to two ounces more than their age. A reasonable bladder stretch goal for teens and adults is about sixteen ounces (two cups or nearly half a liter.) Some people feel full at as little as half the desired volume. By doing five to seven bladder stretches a week, a ten-year-old who feels full at less than eight ounces, and hasn't used this exercise before, can probably increase 'full' and 'stretched' volumes by about an ounce a month over three months.
An increase in the difference between 'full' and 'stretched' can help reduce daytime urgency wetting, but increasing the volume at 'full' may make more difference in how soon a sleeping person's bladder empties itself. The first objective is not to delay sleepwetting all the way until morning, but just past the first two and a half or three hours of deepest sleep. If the bladder doesn't reach full until after the sleeper passes into a period of lighter and REM/dream sleep when most sleepers are more easily awakened, then the use of alarms and/or imaging exercises is more effective.
I am NOT a medical professional and do not offer advice that should be taken as medical or therapeutic in intent. Always consult a doctor for medical diagnoses and treatments. I have researched the area of teaching bladder control and managing bladder disabilities as part of volunteer work with incontinence support organizations and families of children with disabilities. I am a writer, actor, storyteller, children's bookseller, and parent of two young adults. I serve as list owner of the e-mail lists EnuresisKids [moderated] and EnuresisParents.