If you've ever noticed you're thirstiest right before you slip under the covers, it's not in your head. Your urge to chug water may actually increase in the moments before you call it a night, finds a new study out of Canada's McGill University. At least, that's the case with mice. Researchers now know that rodents take in a lot more water two hours before they sleep, even if they're fully hydrated. The McGill scientists wanted to know why that is, so they deprived one group of animals of fluids before bedtime and let another other group imbibe at will. The results? When the mice woke up, those that didn't drink were far more dehydrated than those that did. That may sound like an obvious conclusion, but the impulse behind it is more subtle: "The increased thirst at bedtime serves to motivate water intake in anticipation of this, so the body has a water reserve that protects against overnight dehydration," says Charles Bourque, lead author of the study. Here's the theory: Your brain knows you might grow dehydrated while you snooze, and it tries to convince you to do something to prevent it. The secret weapon it deploys is a hormone called vasopressin. Produced by a brain region that regulates your circadian rhythm—the body's internal clock—vasopressin also stimulates those thirst neurons.
But let's remember: This study was on mice, not people. Though you and a rodent have morein common when it comes to these nighttime cravings than you might think: "In all likelihood, many of the findings apply to humans," says Bourque, who adds that more research is needed before he can reach any definitive conclusions.
In the meantime, he says, if you scale the numbers from the mouse studies, a large glass of water should be enough to keep you hydrated all night—hopefully without having to get up in the middle of it.