Your body works around the clock to maintain a normal internal temperature. When temperatures climb into the 90s—or even above 100°F—and humidity is high, your natural cooling system must work overtime.

You begin sweating more. Blood is redirected toward the skin to release heat, your heart works harder to circulate blood, and high humidity makes it more difficult for sweat to evaporate. As a result, your body becomes less efficient at cooling itself.

All of this requires energy.