BY THE OPTIMIST DAILY EDITORIAL TEAM

A day at the beach should leave you refreshed, right? But somehow, even after hours of lounging on a towel, soaking up the sun, and dipping your toes in the ocean, you feel completely wiped out. If this sounds familiar, you’re not alone. Turns out, there are some surprising scientific reasons why a beach day can leave you feeling totally spent.

Heat takes a toll on your body

First up: the heat. Most beach days happen under the blazing summer sun, which pushes your body into overdrive. “Humans need to be able to regulate our body temperature,” says Craig Crandall, a physiologist at the University of Texas Southwestern Medical Center. Unlike lizards, we need to stay close to 98.6 degrees Fahrenheit to function well.

Staying cool takes real energy. “People exposed to heat consume slightly more oxygen,” says Crandall, which means our cells are working harder. This increase in effort can quietly wear you down over time.

Your heart also plays a starring role in temperature regulation. When it’s hot out, your body sends more blood toward your skin to help shed excess heat. But that means your heart is pumping more and faster. Crandall explains that a person with a typical resting heart rate of 60 beats per minute might see that number spike to 100 or 110 in the heat. While that’s not dangerous for most healthy people, it’s definitely more taxing than it feels.

Don’t forget dehydration

Sweating is your body’s natural air conditioner, but it also leads to fluid loss, and most beachgoers don’t drink enough water to compensate. “As we get dehydrated, we will get symptoms not too different from heat exhaustion,” says Crandall. You might feel sluggish or moody, even if you’ve been lounging all day.

Even mild dehydration can impact mood, alertness, and thinking. And if you’ve been sipping cocktails or cold beers by the sea, the effect can double. Alcohol is a diuretic, meaning it causes you to lose more fluids. Plus, it can dull your awareness of how dehydrated or overheated you actually are.

Surprise: beach days are more active than you think

Even if your intention is to chill, beach visits sneak in a lot of movement. Walking across sand while hauling a cooler, playing beach games, wading in the surf, or wrangling kids all add up. Compared to a typical desk-bound day, a beach outing is surprisingly active. “If I go to the beach, it’s orders of magnitude more activity over hours than what I do day-to-day,” Crandall says. So it’s no wonder your body feels the burn later.

A natural nap response

Interestingly, some researchers think that drowsiness in the heat may be wired into our biology. A 2022 study on fruit flies found that heat triggers brain signals that push them to nap, kind of like a Spanish siesta. We don’t yet know if humans respond the same way, but it’s an intriguing idea.

Then there’s the sun itself. Even if you don’t burn, exposure to sunlight can wear you out. Sunburn, as Crandall notes, causes inflammation, which can trigger whole-body fatigue. But even without the redness, sunlight alone may increase mental fatigue, according to a 2004 study. A 2021 study found similar results in outdoor workers whose exposed skin led to more symptoms of heat stress, even when temperatures stayed the same.

Why exactly this happens is still being explored. Crandall points out that sun exposure sparks chemical reactions in the skin like vitamin D production, and it’s possible that other hormones or signals are at play too.

How to recover from a beach-induced energy crash

So what’s the best cure for post-beach fatigue? Crandall recommends two essentials: stay hydrated and avoid sunburn. Taking shade breaks or heading indoors during peak sun hours can help too. But if you’ve covered the basics—water, food, sunscreen—then maybe your tired body is telling you something.

“I would say, just take a good nap,” says Crandall. Preferably under an umbrella, with the ocean breeze as your soundtrack.