Here’s Why a Day in the Sun Makes You Tired

There's nothing wrong with a little fun in the sun. But if you're wiped out afterward, that's probably a red flag. Here's what you need to know.

Temperature records are being shattered all over the United States this summer. Yet as this heat intensity rises, beaches and parks aren’t the only areas filling up with people — emergency rooms are, too.

Ever notice how the sun makes you tired after a day spent outdoors? The Mayo Clinic tells us this sleepiness is an early sign of heat exhaustion, a condition that can escalate to life-threatening heat stroke. A 2021 study published in The BMJ links today’s increasingly hot summer days with more hospital visits for adults of all ages. 

Now, sun exposure has plenty of benefits. It keeps your sleep cycle sound, regulates 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 them. 

Dehydration from the Sun Makes You Tired

Whether you’re sitting poolside or hiking a mountain trail, the sun dehydrates your body, according to the National Sleep Foundation. And that overwhelming fatigue after a summer day outdoors 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.

The authors point out this dehydration risk doesn’t only apply to people performing intense activity in the heat, either. Simply catching some rays on your favorite beach towel causes your body to sweat, draining fluids and salt that keeps your risk for heat exhaustion or heat stroke at bay.

Be mindful of your fluid intake throughout the day and be aware of dehydration symptoms, such as stinky breath and irritability. You can also keep nutritionist-approved electrolyte drinks on hand to replace the salts you lose from sweating.

The Sun Makes Your Body Work Harder

All that extra time in the heat causes your body to work hard, no matter what you’re doing. The National Sleep Foundation says our bodies work hard throughout the day to maintain a constant internal temperature. So when we expose ourselves to long periods of heat, our bodies go into overdrive to maintain that core temperature. 

That Suntan Makes You Tired

Being in the sun causes chemical changes in our bodies. 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, the same chemicals that tan our skin can also cause fatigue. 

To fight back, The National Sleep Foundation recommends staying inside during the hottest hours of the day — typically between noon and 3 p.m.— and taking frequent breaks in the shade at other times. Balance your activity balanced with plenty of rest, and don’t forget to pack along a sun hat or 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 afterward.

The Healthy
Originally Published on The Healthy

Leslie Finlay
In addition to The Healthy, Leslie has written for outlets such as,,, and more, specializing in content related to healthcare, nutrition, mental health and wellness, and environmental conservation and sustainability. She holds a master's degree in Public Policy focused on the intersection between public health and environmental conservation, and an undergraduate degree in journalism. Leslie is based in Thailand, where she is a marine conservation and scuba diving instructor. In her spare time you'll find her up in the air on the flying trapeze or underwater, diving coral reefs.