In a way—yes.

Your brain is made up of about 75% water, and when you don’t drink enough, it actually begins to shrink slightly. MRI scans have shown that even mild dehydration can cause the brain to contract, pulling away from the skull just enough to cause tension—and often, a headache.

This “sag” also slows down your brain’s ability to process information, concentrate, and react.
Fortunately, rehydration restores the brain’s normal size and function fairly quickly.

It’s not dramatic, but it’s real—and a very good reason to sip more water throughout the day.

The old “8 glasses a day” rule isn’t wrong—but it’s just a general guide. Actual needs vary based on your size, activity level, diet, and climate.

Experts recommend about 11.5 cups a day for women and 15.5 for men, including fluids from food.

A simple rule of thumb: Drink when you’re thirsty and aim for light yellow urine.