In short, it’s your body’s way of letting you know that it’s working a lot harder than normal, and that it’s having trouble delivering blood to your muscles.
“When you’re exercising in the heat, your core temperature goes up and it’s more difficult for your body to cool itself down,” says Dr. Laura Goldberg, medical director of the Cleveland Marathon. As a result, your sweat level increases—which, unless you’re replenishing regularly, can lead to salt loss and dehydration.
“It depletes the fluid and electrolytes in your body, which makes it more difficult for blood to flow to your muscles,” Goldberg says. “That’s why your legs feel so heavy—they literally aren’t getting enough blood to keep them working optimally, so they start to shut down.”
Your brain also needs steady blood flow to keep you alert and awake. Without it, it’s normal for you to feel tired and crave a snooze.
You can prevent some of this sluggish feeling, however, by going into your workout fully hydrated. That doesn’t just mean chugging a few pints of water immediately beforehand. Instead, drink plenty of fluids throughout your day and for several days before a big event—aiming for at least 64 ounces of fluids a day, or more if you’re doing a lot of sweating.
A small amount of caffeine before your workout can perk you up, too, but stick to the amount in one or two cups of coffee. Since caffeine has a slight diuretic effect, drinking more than that could be counterproductive.
Getting your body used to exercising in hotter temps can also help you fight off that tired feeling. “The acclimation period is generally thought to be about 10 to 15 days, but we see physiological responses in as little as five to six,” says Goldberg. “Your blood becomes a little more diluted, you can hold onto a little more water, and your thermoregulatory system starts to kick on at a lower temperature.”
Your other best defense against heat-related tiredness is simply to stay as cool as possible. Beat-the-heat remedies like running early in the morning, choosing shaded routes, dumping water on yourself, and wearing a light-colored hat and clothing can all help you sweat less and decrease your fluid loss. Wear sunscreen too, since sunburn makes it harder for your body to regulate its temperature.
You may also notice that a workout in the heat—even an easy one—leaves you wiped out for the rest of the day. That’s related to dehydration, too: when your blood has less fluid, your blood vessels have to constrict and your heart beats faster to maintain constant pressure.
“You may feel like you didn’t exercise that hard, but your heart worked a lot harder than normal and your muscles were more stressed,” says Goldberg. “Even just sitting outside for an hour can drain you, so if you add exercise on top of that you’re definitely going to feel the effects.”