Here’s Why a Day in the Sun Makes You Feel Exhausted, Says Science

It’s normal for a day out in the sun to make you tired— but if you're wiped out afterward, that's a red flag.


Temperature records are shattering in all corners of the United States this summer. Yet as this heat intensity rises, it’s not just the beaches and parks filling up with people after a dose of vitamin D—emergency rooms are, too. Ever notice how the sun makes you tired after a day spent outdoors? The Mayo Clinic tells us that this sun-induced sleepiness is an early sign of heat exhaustion, a condition that can escalate to life-threatening heat stroke. And a 2021 study published in The BMJ links today’s increasingly hot summer days with a greater rate of hospital visits for adults of all ages. 

Now, sun exposure has plenty of benefits: it helps to keep your sleep cycle sound, regulate your mood, and may even boost your lifespan. But research points to some important reasons why the sun makes you sleepy, too—and why you shouldn’t ignore it. 

Our bodies need sunlight—but just how much? Check out this new research on what amount of sun is best for your health

Dehydration from the sun makes you tired

Whether you’re sitting poolside or hiking a mountain trail, the sun is dehydrating your body, the National Sleep Foundation says. And that overwhelming sense of fatigue after a summer day out is one of the tell-tale signs your body’s fluid balance is out of whack. The danger this poses goes beyond feeling extra thirsty: a 2019 review published in Nutrients warns that losing just two percent of your body water messes with your skin health, mood, memory, concentration, kidneys, gut, and even weight management. (Don’t miss these 7 tasty ways to stay hydrated this summer besides just drinking water.)

The authors point out that this dehydration risk doesn’t only apply to people performing intense activity in the heat, either.  Even a relaxing afternoon of catching some rays on your favorite beach towel is causing your body to sweat, losing the fluids and salt that help keep your risk for heat exhaustion or heat stroke at bay. Be mindful of your fluid intake throughout the day and be sure to remain aware of other dehydration symptoms, such as stinky breath and irritability. You can also keep these nutritionist-approved electrolyte drinks on hand to replace the salts you lose from sweating.

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The sun makes your body work harder

All that extra time in the heat is causing your body to work hard, no matter what kind of fun in the sun you’re enjoying. The National Sleep Foundation explains that throughout the day, our bodies work hard to maintain a constant internal temperature. So, when we expose ourselves to long periods of heat, it has to work in overdrive to maintain that core temperature. 

If you do start to feel overheated, here are some expert tips ways onto keeping cool all summer long.

All that extra time in the heat is causing your body to work hard, no matter what kind of fun in the sun you’re having. The National Sleep Foundation explains that our bodies work hard to maintain a constant internal temperature, and when we expose ourselves to long periods of heat, our body has to work in overdrive to maintain that temperature. When you do start to feel overheated, here are some ways to cool down and fast.

That suntan makes you tired

Being in the sun causes chemical changes in our body—just look at your suntan (or sunburn) for proof and those UV rays cause some serious damage. When we spend too much time in the sun, these same chemicals that make our skin tan can also cause fatigue. 

To fight back, The National Sleep Foundation recommends you avoid heading outdoors during the hottest hours of the day— typically between noon and 3 p.m.— and take frequent breaks in the shade. Keep your activity balanced with plenty of resting, and don’t forget to pack along a sun hat or an umbrella for additional cover. If you limit your sun exposure and remain mindful of possible sunburn, you should be able to spend a day outdoors without feeling like you need a nap to recover from it. (And you can still walk away with a great, golden tan with one of these top-selling self-tanners.)

Follow The Healthy on Facebook, Instagram, and Twitter for the latest health and wellness updates all summer long. And check out these sunny summertime reads:

Websites: The Mayo Clinic: “Heat exhaustion”  The National Sleep Foundation: “Why Does the Sun Make You Tired?”   Journals: The BMJ: “Ambient heat and risks of emergency department visits among adults in the United States: time stratified case crossover study” (2021) Nutrients: “Narrative Review of Hydration and Selected Health Outcomes in the General Population” (2019)

Leslie Finlay, MPA
In addition to The Healthy, Leslie has written for outlets including Buzzfeed News, VeryWell Fit, and, specializing in content related to healthcare, nutrition, fitness, and mental health. As a lifelong athlete and instructor, she’s passionate about learning and communicating the latest in health and wellness..