How much water should you drink? Eight glasses a day? Ten? The Mayo Clinic says that men should drink thirteen cups "of total beverages" every day, and women nine. But, really, you should just drink when you’re thirsty. It turns out your body is pretty good at judging when it's low on water. In fact, drinking when you’re not thirsty might even confuse your brain.
However popular these X-cups-per-day rules of thumb are, researchers say they're actually not based on empirical evidence. A recent study in the Proceedings of the National Academy of Sciences looked at how the brain regulates the sensation of thirst and the reflex that allows you to swallow and found that the brain actually tamps down on that swallowing reflex once you’ve had enough to drink. Drinking when you’re not thirsty forces you to override that instinct. "It's a biological mechanism in quite ancient parts of the brain, and it seems intent on stopping you from drinking too much," Michael Farrell, the study’s co-author, told Australia Public Broadcasting.
To watch people’s thirst in action, Farrell and his team put nineteen people in a room and had them exercise. By the end, the participants had lost about one percent of the their body weight in water from sweating, and they were thirsty. So the researchers placed them into an MRI scanner and gave them water, watching their brains as they hydrated. They were then asked to keep drinking, even after they were no longer thirsty. As it got less and less enjoyable to drink, their brains revealed a few interesting things about regulating thirst.
First, it became harder and harder for the subjects to swallow, and the motor cortex that controls swallowing had to work harder to make it happen. Then areas of the brain that are known to inhibit swallowing began to come alive, like the amygdala and the periaqueductal gray—a region of gray matter in the middle of the brain. "Our interpretation is that the PAG and amygdala were sending messages to the swallowing switch to prevent it turning on," Farrell told ABC. "Consequently, the motor cortex had to send stronger than usual signals to turn the switch on."
So, forget the strict thirteen glass water regimen, and stop forcing yourself to drink when you’re not thirsty. Your brain has many years of evolutionary training to help you figure out when to drink, and how much, and you should just listen to it.