Your body’s efforts to cool down use up a lot of extra energy
Living through history can be exhausting, especially when that history concerns heat waves. My kids and I have spent the last few days scrambling to avoid the record-setting temperatures that recently cooked the Bay Area. While we thankfully avoided serious health concerns like heat stroke or severe dehydration, there were plenty of moments where the most sensible course of action seemed to be to lie on the ground in front of a fan and hope for the best. Part of this was psychological, but part of it was because living in extreme heat is simply hard work for your body. While it’s a passive process for the most part, sweating buckets can burn some calories, even without any exercise.
Extra work for overheated endotherms
Your body has a few tricks it employs to try and control your temperature. When it’s cold, you’ll likely notice things like shivering to get your muscles to generate some heat, but cooling down can sometimes be more subtle. For example, your body will dilate blood vessels to get more blood near the surface of your skin, leaving you looking a little pinker than usual. That blood is carrying more heat towards the outer surface of your body to keep your core cooler, and ideally let the surrounding air wick some warmth away. Many animals have special anatomy to maximize this trick, but humans have to make due with activated blood vessels, which is more work than when you’re feeling cool.
Even if you don’t actively choose to sweat, it’s not an effortless process. Your body is rerouting water to your skin for a very effective cooling process, but it has to boost your metabolic rate to do so. This means that your heart works harder, and you may push yourself closer to dehydration to cool down. Dehydration alone can cause fatigue, so it’s easy for it to wear you out when you’re drying out to cool down.
Finally, all this heat can require actual repairs. Ultraviolet light from the Sun can of course cause damage to your skin, and while a bad burn may feel like it just sits there for ever, your body starts trying to repair itself right away. In addition to trying save cells, burned skin also demands more of your body’s water reserves, increasing your dehydration risk even more.
Help yourself beat the heat
So feeling tired at the end of a hot day makes sense, even beyond the way that intense heat can sap your will to function. If you can’t just take a siesta until things start to cool off in the evening, consider assisting your body in its efforts to lower your temperatures. Aside from moving to a cooler environment, drink lots of water to avoid dehydration, throwing in some salty snacks to help retain that water. You can also help maximize the effect of your heat-pumping blood vessels by trying to cool your blood as it circulates, putting a cool, damp washcloth on your neck, wrists, feet or other key anatomy. You probably won’t feel fresh as a daisy at the end of the day, but it should help you get the most from the effort your body is putting into keeping you cool.
Source: Why Does Being in the Heat Make Us Feel Tired? by Laura Geggel, Live Science