The idea that you’re supposed to drink eight glasses of water every day to stay hydrated is a myth. First of all, there isn’t a set amount of water all humans need to drink. Each person’s water needs differ, depending on their health, their size, and many other factors. As we’ve written before: If you find yourself confused about whether or not to drink eight glasses of water a day, chances are you’re overthinking it. There’s a perfect system that tells you when you need to drink, and it’s called thirst.