You won’t go a day in the hot summertime without someone reminding you to stay hydrated, but where’s that same caution in the wintertime? After all, the combination of colder weather, more dry air, and indoor heat actually create a condition in which people are chronically dehydrated.

In fact, experts say some 75 percent of people are dehydrated during the wintertime and have no clue. This is a problem. Being hydrated does so much more than quench your thirst: it gives your body the ability to fight off infections and flush out toxins via urination. Water also carries oxygen to the brain and provides essential nutrients to all cells in the body. And when you’re dehydrated, especially in the wintertime, it can cause you to feel low on energy and lead to mood fluctuations.

“Hydration is a key element to maintaining a healthy immune system,” says Jessica Bippen, a St. Louis based Registered Dietician. “Our immune system is highly dependent on the nutrients in our bloodstream and adequate hydration allows for these nutrients to flow.”

So, how much water should you be drinking on a winter day? Bippen says it depends on bodyweight.

“There are guidelines, for example about ½ your body weight in ounces of water per day. But the truth is that we are all unique. Different factors play a role in how much water your body needs on a daily basis. You know you’re drinking enough water if your pee is light yellow or clear.”

We know our readers are smart enough to remember to stay hydrated, but with this article, we just wanted to highlight how essential drinking more water is during the colder months—especially since hydration boosts immunity and helps fight infection, which is more important than ever right now.