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Why Am I So Tired After a Day in the Sun?

July 30, 2025

You spent the day relaxing at the beach — so why does it feel like you just ran a marathon?

Whether it’s a backyard barbecue, a hike, or just a sunny afternoon on the patio, a sunny day tends to leave us all feeling the same way: totally wiped out.

But why do so many of us end up tired after a day in the sun?

“Sun exposure affects your body in more ways than you might think,” says Anthony Sampino, DO, a family medicine specialist with Hartford HealthCare Medical Group. “Feeling tired after a day outside is normal, and there are a few reasons why.”

1. You’re losing too much water.

Even if you’re not sweating buckets, being outside in the heat causes your body to lose fluids — fast.

“Dehydration is one of the most common reasons people feel exhausted after being in the sun,” says Dr. Sampino. “Even mild dehydration can cause fatigue, headaches and brain fog.”

And it’s not just water loss — your body is also shedding electrolytes, like sodium and potassium, which help regulate energy and muscle function.

Try this: Drink water all day, not just when you’re thirsty. And if you’re active or sweating, consider drinks with electrolytes — but watch out for added sugar.

2. Your body is working harder than you think.

You may not realize it, but keeping cool takes a lot of work.

“When you’re out in the sun, your body is constantly trying to regulate your temperature,” says Dr. Sampino. “Your heart is pumping more blood to the skin, your sweat glands are working overtime — it’s like your internal air conditioner is running full blast.”

Even if you’re not physically active, your body is burning energy behind the scenes to stay cool, which can leave you feeling wiped out later.

It’s another big reason people end up tired after a day in the sun — your body is putting in extra effort even when you don’t realize it.

Try this: Give your body a break by spending time in the shade, wearing breathable clothing and taking short, regular rests indoors or in cooler spots. Even a few minutes out of the heat can help you reset.

> Related: How Summer Heat Affects Your Heart

3. You didn’t eat enough.

Hot weather and sunshine can suppress your appetite — but your body still needs fuel.

“People often eat less when they’re out in the sun all day,” says Dr. Sampino. “But your body is using more energy than usual, and without enough calories, you’re going to feel sluggish.”

Try this: Pack light, nutrient-rich snacks like fruit, trail mix or sandwiches if you’re going to be outdoors for a while.

4. Sunburn fatigue is real.

A bad sunburn doesn’t just hurt — it can also make you tired.

“When your skin is burned, your immune system kicks in to repair the damage,” says Dr. Sampino. “That immune response can feel a lot like being sick — low energy, achy, sometimes even chills or nausea.”

Try this: Always wear sunscreen with SPF 30 or higher, reapply every two hours, and take breaks in the shade when you can.

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Sunny days are great, but they aren’t always restful.

If you’re feeling unusually tired after a day in the sun, it’s not in your head. Your body is working hard — staying cool, hydrated, and protected — even if you’re just relaxing.

The good news? A little preparation can help you enjoy the sunshine without the crash.

“Just being mindful of how your body responds to heat and sun can make a big difference,” says Dr. Sampino. “Stay hydrated, take breaks in the shade, and don’t ignore the signs of fatigue — they’re your body’s way of asking for a little help.”