Arginine vasopressin is a hormone with a wide range of duties. For instance, in older children and adults, its level in the blood rises at night when the hormone decreases urine production. That relieves most of us of the burden of rising in the middle of the night to pee. (Children's vasopressin levels don't cycle when they are young, which is why nighttime diapers are as soggy as daytime ones and, to some degree, nighttime dryness comes slowly to some kids.) Arginine vasopressin also influences blood pressure, blood clotting, the body's response to stress, and even signals nausea in monkeys and humans. In mice it plays a role in controlling aspects of social behavior.