Why Does the Sun Make You Sleepy?

Emily Boynton Fact Checked
Woman lying in the sunshine
© Trinette Reed / Stocksy United

This summer, the Pacific Northwest has been hot. As in, record-setting, triple-digit temperatures, lay-on-the-floor-in-front-of-a-fan kind of hot. 

Whether you’re basking in the glow or would prefer to skip the season altogether, soaking in all these rays affects how your body functions and can zap energy. (Think needing to nap after a relaxed day of sunbathing.)  

High temperatures also pose a threat to your health — in May and June alone, there were more than 3,500 heat-related illness emergency department visits in Alaska, Idaho, Oregon and Washington. 

Stay safe on your next summery outing by learning how to maintain your energy, recognize signs of heat stroke and exhaustion, and recover from a day in the sun.  

Why does being in the heat make you tired?  

When you spend a day on the water or at the park, your body is working overtime to keep you from overheating. 

“A lot of tiredness stems from dehydration and vasodilation,” says Dr. Ashwin Rao, a family and sports medicine doctor at Sports Medicine Center at Husky Stadium

Vasodilation, the widening of blood vessels in your skin, allows more blood to flow to your body’s surface, causing you to flush. This helps your body dissipate heat and maintain its regular temperature, but it also means there is less blood available to flow to other parts of your body.  

Similarly, when you’re hot, your body moves some core fluid to the outside of your body (aka sweat) to cool you down. However, this loss of core fluid can lead to dehydration.  

While these processes are effective at keeping you cool, they tax your body and can leave you feeling fatigued.  

“You lose some of your core fluid and blood circulating in your gut and brain because your body is instead trying to work on cooling you down,” Rao says.  

As your body’s resources are split between maintaining an appropriate temperature and keeping vital processes functioning, you are left with less fluid and fewer calories to make you feel energetic.  

What are the differences between regular tiredness and heat exhaustion or heat stroke? 

Feeling tired from heat exposure isn’t ideal, but it’s ultimately not a major risk to your health.  

However, heat-related illnesses like heat exhaustion and heat stroke, in which your body can no longer regulate its temperature, can be life-threatening.   

Rao notes there are a few signals that your sun sleepiness is tipping into a more dangerous condition.  

“The easiest thing to monitor is your urinary output,” he says. “If you realize you haven’t peed all day, it’s a sign that things are amiss.” 

Other red flags include increased heart rate for a prolonged period, which indicates your body is having a tough time regulating heat; and if you stop sweating, which means your body is no longer helping you cool down. Children and older adults are more likely to experience these symptoms. 

If a friend stops sweating or becomes nauseated, dizzy or confused in the heat, they are experiencing a medical emergency.

Move them to a cool area, call 911 and try to loosen their clothing to cool them down while you wait for the ambulance. If they are responsive and able to drink, you can also give them water.  

How can you maintain energy during a day in the sun? 

Avoid succumbing to summertime sleepiness and enjoy your fun in the sun with these safety tips.  

Drink water  

Hydration is important before, during and after your time in the sun. 

Upping your water intake ahead of time prepares your body for loss of fluid and will help you maintain homeostasis (your body’s balanced, baseline state) in the heat; hydrating day-of maintains your body’s fluid stores and optimal functioning even as you sweat. Afterward, water will help you recover.  

As for sports drinks? The answer is generally no.  

“The sugar content in those drinks will often pull fluid out of your soft tissues and into your gut, so they don’t hydrate you as well as you would think,” Rao says.  

Coffees, teas and juices can be refreshing, but they are also often high in sugar and won’t help offset dehydration. (Plus, chances are you are less likely to drink water if you’re busy sipping on a venti cold brew.) 

If you must have a flavored beverage, aim for options with less sugar and more electrolytes, like pediatric fluid repletion drinks. 

Fuel up mindfully 

Some summertime snack favorites might be contributing to your energy drain.  

While s’mores, hotdogs and hamburgers are delicious, the heavier foods will sit in your gut and cause your body to send more resources and expend more energy on digestion.  

Instead, opt for lighter fare on a hot days — Rao recommends salmon for an energy-boosting PNW favorite. 

Avoid excessive alcohol 

You’ve heard it before, but it’s worth mentioning: Alcohol promotes dehydration.  

“You want to avoid things that are going to dehydrate you, and alcohol is a diuretic, which means you lose more fluid if you drink alcohol,” Rao says. 

If you do plan to sip on a cocktail or spiked seltzer, be sure to continue drinking water as well so that you replenish your fluids.  

Bring sun protection 

Blocking out sunrays and direct heat — via sunscreen, umbrellas, tents, hats or shade from trees or buildings — will help your energy levels and keep you safe.  

You'll also want to reapply sunscreen every two hours for optimal protection.  

Listen to your body 

Summers in Seattle are hard to beat, and favorite activities like paddle boarding on Lake Union or picnicking in Golden Gardens aren’t to be missed. But it’s equally important to know when it’s time to take a break. 

“Heat can be dangerous and excessive heat can be excessively dangerous,” Rao says. 

Before exercising outside, check the heat index on your phone or computer to make sure it’s not in a dangerous zone for being active (the National Weather Service notes you should take caution in temperatures as low as 80 degrees Fahrenheit with high humidity and extreme caution in temperatures above 90 degrees Fahrenheit). You should also pay attention to air quality, especially in the later summer months when wildfire smoke is common, Rao says. 

When you are outside, be mindful of the cues your body gives you, like feeling thirsty or needing to take a break from activities. If you start to feel dehydrated or dizzy, move to the shade, hydrate and let yourself rest.  

Sunny weather recovery tips 

Along with taking care of yourself while you’re in the sun, it’s important to help your body recover afterward. 

Rao notes two main ways to do this: hydrate and sleep. 

“If you had a busy weekend and were doing a lot, plan for recovery days if possible,” he says.  

Continue to sip on water when you get home and make sure to get adequate sleep (7-8 hours for most adults). Taking it easy will help your body return to a relaxed, baseline function. 

That way, you’ll be ready to go for your next sunny weather adventure.