A little bit of sunshine has its perks: an influx of mood-boosting vitamin D, and ideal weather conditions for outdoor activities, for starters. It also has its setbacks: sunburn and that completely wiped, must-nap-now feeling.

We all know what causes sunburn. But what is it about the sun that’s so damn draining?

Turns out, there are a bunch of reasons that the rays make you want to conk out on the couch. Some of them are simple biology, but others are more controllable. Read on to find out why a sunny day makes you so sleepy—and what you can do to revive your energy, fast.

Reason why you’re tired: You’re dehydrated

When you’re hot—as is likely the case if the sun’s beating down on your back—you sweat more, making you lose water and electrolytes faster than you normally would, says Sogol Javaheri, M.D., an internist and sleep medicine doctor at Brigham and Women's Hospital in Boston, MA.

That can leave you dehydrated, which can be exhausting. That’s because dehydration can increase the thickness of your blood, which means your heart has to work harder and faster to get blood to your brain.

So make it your business to drink up—just avoid beverages that contain caffeine or are alcoholic, since they can make dehydration worse.

Related: 5 Surprising Sources of Caffeine You Never Even Thought Of

But chugging water isn’t going to be enough, says Dr. Javaheri. You also want to snack on something salty, like a pickle or a few sips of a sports drink, to replace the sodium you’re losing. Sodium’s important, since it helps regulate your body’s fluid balance.

So how do you know you’re getting enough? There’s no one-size-fits-all recommendation for fluid intake, so keep an eye on your pee to tell how you’re doing. You want your stream to be a light-yellowish clear. (Find out what else the color of your pee says about your health.)

Reason why you’re tired: Your body’s working overtime

It’s not just sweat that contributes to sun-related fatigue, though. Heat from the sun raises your body’s internal temperature, says Melissa Levin, M.D., a dermatologist at Marmur Medical in New York City.

“Your body is always working—whether it’s hot or cold out —to maintain a body temperature somewhere in the range of 98.6 degrees. So the hotter it is outside, the harder your body is working to maintain its temperature,” says Dr. Javaheri. “That’s exhausting. It uses a lot of energy and increases your body’s metabolism.”

Your move, then, is to do what you can to keep your body cool—a difficult task during the hottest days of summer. But a little proactive planning can help.

“Keep in mind that the sun's ultraviolet radiation is strongest from 10 a.m. to 3 p.m.,” says Dr. Levin. So minimizing activity outdoors at that time is key to preserving your energy. If you must be outside then, seek shade, which can help you from overheating and feeling drained.

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Reason why you’re tired: You got sunburned

Uh oh, you screwed up: Whether you skipped on sunscreen altogether, or made one of these 7 surprising sunscreen mistakes, you now are nursing a sunburn.

Besides feeling super painful, your burn is likely exhausting you, too. That’s because sunburn itself also raises your body’s temp and causes dehydration, too. One of the big symptoms is fatigue, says Dr. Levin.

Your best bet is prevention here. If you’re avoiding sunscreen because you think it feels gross or takes too long to apply, we’ve got you covered—check out the 7 best sunscreens for every type of guy. If you’re already fried, make sure you drink more extra water than usual, since that can help hydrate your skin from the inside out. Then try these 5 sunburn relief products to soothe your tender skin, too.

Reason why you’re tired: You’re simply doing more than usual

Of course, it’s also true that when we’re on vacation or simply spending a few hours in the sun, we’re more likely to be active—paddle boarding, running on the beach, or swimming, instead sprawled on a couch.

When we’re active, our bodies create more of a chemical called adenosine, which makes us tired, says W. Christopher Winter, M.D., Men’s Health sleep advisor. To this extent, he says: “An accountant will never feel as sleepy as a world class athlete.”

Couple activity with the other draining effects of sunshine, and it’s no surprise you’re ready for some shuteye. So if you need to be clear-eyed and energized for hours on end, you might want to switch your morning beach run to an under-the-stars affair. (Or get your workout in at home instead. Try The 21-Day MetaShred—you'll burn fat and build lean muscle.)

Reason why you’re tired: Nature kicks in

Being in the sun all day and being outdoors when the sun sets just naturally makes you tired, too. It’s why people who go camping feel ready for bed at 8 p.m., says Dr. Winter.

“Being in the sun suppresses [the sleep hormone] melatonin, and then when the sun goes down, your body produces melatonin,” he explains. This effect is more noticeable and pronounced when you spend your waking hours in nature rather than under the florescent lights of an office.

Can’t argue with nature. Your body will want you to hit the hay a little earlier than normal if you’re been outdoors on a sunny day—it’s nothing to worry about, he says. But if you just must keep going, try to keep light up in the evening with bright lights indoors, he says. “Boost your light artificially rather than going to dark restaurant.”

Related: The 7 Worst Things You Do When You Can't Sleep