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How come you wake up at night to urinate but not to do anything else?

We’re often too embarrassed to inquire about the strange facts of our internal plumbing, but the answer is just plain biology. The sophisticated, intelligent neurons in your gut that control colon contractions, which push out waste, are also influenced by your body’s circadian rhythm, the internal clock that wakes you when it’s light out and makes you feel sleepy at night. So most people don’t have the urge to empty their colon in the middle of the night. On the other hand, the bladder, which acts a reservoir for the continuous flow of urine produced in the kidneys, can stretch only up to a certain volume before you gotta go. Normally, you can sleep six to eight hours without having to urinate, but certain medical conditions or drinking too much water before bed can wake you to use the bathroom at night.—Pankaj J. Pasricha, MD, director of neurogastroenterology at Johns Hopkins Center for Neurogastroenterology