what causes frequent urination during nightime?

The doctor may have ordered a blood test to make sure the child's kidneys are functioning properly. Also, see:

Nocturia (Nighttime Urination): Is It Nighttime Incontinence?

http://www.healthcentral.com/incontinence/c/45/12997/nocturia

Childhood Enuresis (Bedwetting)

http://www.healthcentral.com/incontinence/c/45/1799/enuresis

How to Discuss Enuresis (Bedwetting) with a Child

http://www.healthcentral.com/incontinence/c/3328/3568/child-part-1

I hope this is helpful. Good luck.  - MG

There are many theories about what might cause children to wet the bed at night, or in this case, to wake up frequently to urinate. One of the most accepted theories is that the body creates a hormone called vassopressin. At night the body should create a surge of vassopressin that causes the body to condense urine (that's why urine is usually a darker shade of yellow in the morning). Some bodies don't produce that extra surge at night, and so the body doesn't condense the urine and thus you have to wake up more often to go to the bathroom during the night.

Some children also might have a smaller bladder that hasn't "caught up" to their body yet.

Good luck!

My two boys had this problem. They wet the bed EVERY nite until they were about 12 years old. They are both in college now. This is a medical condition where they sleep so soundly that it is impossible for them to get up and "go". There is more to it than that - something triggers the bladder to empty. There IS a medication which can help or eradicate this symptom of the disease/condition. I chose not to give it to them since I was not sure of the side effects. It WAS a pain and I'm sure embarrassing to them both - they wore "pull-ups" at night, every night. But, I made sure NEVER to scold them or humiliate them. EVER. They could NOT help it any more than a child with Turretts, for example, could help a nervous tic. Not sure if I should have given them the pills or not. I would recommend talking to a pediatrician and explore any side effects to balance the harm vs. the humiliation of this sympton. Be assured that there are plenty of boys (almost always affects boys) who who are suffering with this. As a parent, be matter-of-fact, patient and supportive- that's my advice. Oh, and I forgot to mention: when I was wondering aloud to my husband about how long this would go on, he admitted to me that HE had wet his bed until age 12. This gave me a clue as to how long it would last and sure enough, it stopped at age 12 for both boys...just like it did for their dad.  (my girl wet the bed on a pretty occasional basis until age 7 ....doctor said all this is hereditary).  - E