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Why being out in the sun makes you tired, according to experts

On a sunny afternoon two weekends ago, my partner and I took a chill stroll near his parents’ house, moving at a leisurely pace along mostly flat meadows for less than an hour — and returned completely wiped. All I wanted to do the rest of the day was take an epic nap. “Just being out in the sun can take it out of you,” my partner reasoned. I thought back to times when even sprawling out by the pool, barely moving at all, had transformed me into a groggy pile of mush. What gives? I consulted two experts to help me understand why the sun makes you tired.

First, even if you might automatically blame the sun, you can’t rule out other factors like temperature and humidity, points out Ron Weiss, an assistant professor of clinical medicine at Rutgers New Jersey Medical School. “I think it’s a temperature and heat issue,” he tells me.

Indeed, your body constantly tries to maintain an internal temperature of around 98.6 °F, explains Shawn Kwatra, an assistant professor of dermatology at Johns Hopkins University School of Medicine. When it’s hot outside, it needs to work even harder to cool you down and prevent its internal temperature from rising too much. “Your body is sweating and doing other things to help you stay cool, and that increased effort can sometimes make you feel tired.”

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Dehydration can also cause sluggishness, Kwatra says. That’s because when you’re dehydrated — which occurs when you excrete more fluids than you consume — your blood volume is lower. As a result, there’s less blood flow to your brain, which can make you tired. In response, your heart pumps harder in order to supply your brain with more blood, also draining you.

Your lifestyle could also factor into whether the heat makes you sleepy, Weiss says. People who eat mostly whole, plant-based foods and exercise regularly typically don’t have fatigue issues.

But besides the heat, the sun itself can play a role. When you’re in the sun, your body works to repair the skin damage and radiation it can cause, Kwatra says. Blood flow to your skin increases, especially if it’s sunburnt. More blood flow to the skin means less blood flow to other parts of your body, so your heart works harder to pump blood to these regions, which can tucker you out, too.

That said, sun exposure isn’t the enemy. In fact, sunlight has a bunch of health benefits. For instance, the experts I interviewed pointed out that it increases vitamin D levels, important for absorbing calcium from the gut, bone growth, and other biological functions. It also regulates melatonin, a hormone that helps maintain sleep-wake cycles, Kwatra says. Regardless of your skin color though, always stay sunscreened up.

Too much of a good thing can be dangerous. Excessive sun exposure can heighten the risk of not only skin cancer, but also heatstroke, whose symptoms include rapid breathing and heart rate, altered mental state, headache, and nausea, Kwatra says. If you notice these, relocate to a cooler place and rest. To minimize your risk of dehydration and heatstroke, avoid hanging outside during the hottest hours of the day — 10:00 a.m. to 2:00 p.m.— and drinking alcohol or caffeine. Basically, anticipate your body working on overdrive to keep you cool.

No wonder I felt tired after my short walk two weekends ago. My body was literally working hard to counteract the effects of the sun and heat on an early July afternoon. There was nothing wrong with me; my body was simply operating as it was supposed to.