Spend a day at the beach swimming, playing volleyball, or participating in any other physical activity, and you’ll no doubt be exhausted by the end. But spend the entire time lounging around, and you might find yourself just as sleepy.

Why does your body get so tired from simply being in a hot spot, even if it’s doing basically nothing else?

Because, as Live Science explains, it’s actually doing a lot. Your body is always trying to maintain your internal temperature—and when you’re surrounded by heat, that requires significantly more effort.

One vital way your body cools you off is by dilating your blood vessels, called vasodilation. Not only does this increase blood flow, but it also helps blood get closer to the skin, where it can offload some excess heat. And if you think having a bunch of blood barreling along right beneath your skin’s surface seems like it might make you look pretty red, you’re right; plenty of people get flushed when they’re hot for this exact reason.

Sweating, the human body’s most well-known cooling mechanism, is quite labor-intensive, too. Your heart rate and metabolic rate both increase, burning through your energy and potentially leaving you dehydrated.

“In addition to fluid loss, the loss of essential salts and electrolytes such as potassium, calcium and magnesium can also make you feel worn out,” Dr. Aimée Brame, a consultant physician at London Bridge Hospital, told Cosmopolitan.

For the most part, tips for avoiding heat-generated fatigue are self-evident: Stick to shady spots if you’re outside, hop in a cold shower if you’re inside, and stay as hydrated as possible. Fruits and vegetables make great snacks, since they’ll help replace lost fluid and minerals; and a loose-fitting, breathable outfit will let your skin release more heat than something constricting.

Now that you know how hard your body has to work on a hot day, you can freely indulge in lying around without feeling lazy.

[h/t Live Science]