If you notice that you tire more easily in the summer, heat itself isn't necessarily to blame. "Dehydration is what makes you tired," Grandjean says. That's because your body will keep its cells hydrated at all costs, she says, so if you don't replace water lost through perspiration, it will simply take water out of the circulating blood, reducing your blood volume. "As your blood volume goes down, your heart has to work a little harder," she says. "Your body adapts to that by slowing down, and that affects your general feeling of vitality." So your daily 8 or 9 glasses of water become more important on hot days, and, in fact, may not be enough on some days.
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Jessie Szalay is a contributing writer for Live Science. She covers animals, health and other general science topics. Her work has appeared in the Jewish Daily Forward, National Geographic Traveler — Intelligent Travel, Killing the Buddha, Waccamaw Journal and elsewhere. Jessie is finishing her master's degree in nonfiction writing at George Mason University and holds a bachelor of arts degree from Kenyon College. She lives in Washington, ., and is working on a book about the sociological and psychological practice of othering.