How does the bladder transition from releasing urine at night to being able to hold urine at night? I wonder about this. What's the biology of the transition from wetting the bed at night to holding urine at night? Is there a chemical change with how the bladder muscles contract?

I'm not sure how much the bladder has a role in this. Mostly, the reduced frequency of urination during the night is the result of less urine production by the kidneys. This is in response to a hormone called ADH, antidiretic hormone (it has other names as well). ADH acts on the distal convoluted tubules of the nephrons in the kidney to fine-tune the reabsorbtion of water in the tubule bringing it back into the circulatory system. This hormone is released in reponse to a few factors including blood volume. But it's release is also cyclic over a 24 hour cycle. <

It can happen that this daily cycle of ADH release intensity gets out of adjustment and you urinate more frequently at night, perhaps more than in the day. This has happened to me. Also, according to my GP, it is a frequent cause of bed wetting in children.