Why does the sun make you tired
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Why Does the Sun Make You Tired?

Whereas sunny weather can certainly bring on an energetic mood, we also all recognize the feeling of tiredness after a day of making the most of the sun's rays and the need for a summer nap. So why does the sun make you tired?

On cloudy days, we pine for long summer days when the sun seems never to stop shining. Then summer arrives, and we find ourselves fatigued after a few hours of enjoying the weather outside. To help us understand the answer, we share 5 scientific reasons you may feel sleepy or quickly tired after a small part of your day in the sun.

Why do you feel tired after sun exposure?

Tired after sun exposure
Photo by Pixabay

1. Your body is keeping cool

The human body functions healthily at a core temperature of 98.6 degrees Fahrenheit. When the temperature in the environment changes, it disturbs the balance of your body temperature. And your body begins working to restore and maintain normal temperatures. We call this process thermoregulation.

When exposed to excessive heat, your body temperature increases, and your internal thermostat tries to cool you down. Your body moves warm blood to the surface so the heat can dissipate. Your sweat glands also release a mixture of salt and water, which cools you down as it evaporates. 

Regulating high temperatures taxes the body, uses up energy, and increases metabolism. That explains why you notice low energy levels even if you've just been sitting in the sun all day. After a while, you will notice that you start feeling tired and sleepy.

Help your body avoid heat exhaustion

You can intentionally cool down your temperature; that way, you aid thermoregulation and preserve your energy. As you engage in outdoor activities, try to stay under shade, especially during the day's hottest hours. You should wear a hat and breathable clothing. Also, drink cool water and use a fan. 

Caution: The tiredness and weakness felt after a long time in the sun could be signs of heat illness, heat stroke, or heat exhaustion. In such situations, you'll notice cramps in your arms, legs, and stomach, dizziness, extremely pale or sweaty, pale and clammy skin. There may be a loss of appetite.

Heat exhaustion can be resolved in 30 minutes or less with the above cooling techniques. Heat stroke may have set in if body temperature does not cool down within 30 minutes. You should treat it as a medical emergency and get medical help immediately.

2. You are dehydrated

Refilling drinking water from tap
Photo by Bluewater Sweden on Unsplash

Water intake is essential for normal body function. We lose water through sweat and urine, which keep us cool and expel toxins. When we lose more water than we are taking in, dehydration sets in. 

Dehydration upsets the balance of minerals in your body and reduces nutrients circulating through your body. It also means that your body is getting less oxygen than normal. It can cause nausea, dizziness, headaches, and blood pressure drops. This could be why you feel sleepy after a brief time in the sun.

Aside from feeling thirsty, other ways to know you are dehydrated include smelly dark urine, dry mouth, and lightheadedness. Urinating less than four times a day and tiredness are also signs. 

Stay hydrated

If you want to enjoy your time out on sunny days, make sure to keep up your fluid intake. Especially when engaging in a strenuous activity like hiking or exercising. It's best to avoid alcohol and understand that coffee and tea do not count as rehydration measures.

Take an eco-friendly bottle of cool water with you when you go out. Eat plenty of fruits and vegetables like lettuce, cucumbers, and melon. In cases where underlying health conditions aggravate dehydration, rehydration tablets may be prescribed.

If you’re hiking or traveling light, there’s also a great range of collapsible water bottles available today, which means you can fill up as you go without the bulk. 

3. You are sunburned

The sun, warmth, and natural tanning give off ultraviolet radiation. UV rays cause sunburn. Sunburn does more than just turn your skin pink and painful. It can drain your physical and mental strength as well. Moreso, it can cause severe skin damage and even contribute to skin cancer.

When you are exposed to UV radiation, your skin may suffer serious sun damage. Chemical reactions cause DNA mutation to occur2, and the immune system activates to protect you. It increases melanin production, which is what gives a tan. Your blood flow towards the skin surface increases and blood vessels dilate, allowing nutrients and oxygen to fix the damage. That activation could be why you feel sleepy and tired.

Also, sunlight triggers the production of serotonin, which keeps us alert. We can assume that it means that the sun suppresses the sleep hormone melatonin. And when you get out of the sun into a much darker place, melatonin production kicks off in earnest. That causes you to fall asleep very fast. Medical scientists agree that the more serotonin your body makes, the more melatonin it will produce.

Get sun protection

People with darker skin tones may find it hard to tell they have suffered sunburn. That's why it's better to protect your skin from excessive sun exposure, no matter your color. 

Use eco-friendly sunscreen, and wear loose-fitting, long-sleeved clothes and sunglasses that protect from both UVA and UVB rays. Even if you are basking in the sun because you want a tan, try stepping into a room with air conditioning at intervals. 

4. You are exercising

Photo by Kristjan Kotar on Unsplash

We know that a bright sunny day makes you feel energetic and ready to get out amongst it. Your outdoor exercise, like going for a hike, climbing, a long walk, or a full workout, is part of the appeal of warm weather for many. It’s also likely to exhaust you faster exercising outdoors on a hot summer day. There are a few reasons for that.

Even when you exercise in a cool area, you burn energy, resulting in tiredness. It also induces heavy sweating, which makes you dehydrated. Dehydration happens faster when you spend time in the sun, exercising, increasing your body's metabolism and temperature.

As your body tries to cope with increased energy use, low fluid levels, and increased body heat, you will feel tired and a bit sleepy. All of those tasking activities can alter your body's fluid balance and cause mental fatigue, making you feel even more exhausted.

Exercise cautiously

It's not always possible to avoid every kind of exercise because it's hot outside. However, you should engage in strenuous physical activity with caution. You should reduce the length of your exercise and stay hydrated. Endeavor to consult weather forecasts and plan your activities to fall on cooler days. 

5. You are not eating right

With all the energy we expend pursuing summertime activities, refueling our bodies regularly is essential. However, eating a lot is not enough; you need to eat the right kinds of food. Sugary snacks and foods with high salt content are abundant in the summertime but don't provide enough energy. 

The nutrition found in simple sugar foods like breakfast cereals, fruit juice, candy, and processed grains is not sustainable. Those foods increase your blood sugar, and your energy level increases, too. However, you soon exhaust the energy and come crashing down.

Solve this by eating healthier

Don't depend on the sugar content in your favorite sports drink. Eat fresh vegetable salads, fruit salads, fish, and meat to gain more energy. You should consume unrefined whole grains, legumes, starches, and oats. Those foods provide you with a steady supply of energy all day. If you are on the move, iced smoothies or fruits are great options.

Is the sun bad for you?

Knowing that a few hours in the sun can make you tired might have you wondering if exposure to the sun is bad for you. Sun exposure can be beneficial or harmful to your health. It depends on the levels of exposure. 

Medical experts advise that 10 to 15 minutes of sunlight a day supplies vitamin D. The vitamin boosts our immune system and blood cells. It also strengthens the bones and improves the quality of our skin. Spending some time outside can improve your mood as the sun makes your body produce serotonin.

Optometrists recommend at least 3 hours of outdoor activities for children1. So that their eyes receive enough dopamine from sunlight to prevent myopia, sunlight can also improve our moods considerably. So while you should spend time in the sun, too much sun exposure is what you should avoid.

Further reading: 5 Sun Safety Tips To Stay Protected Outdoors


Spending time in the sun is good for you, but you can have too much of that good thing. Protect yourself from UV rays; use a broad-spectrum, water-resistant SPF 30+ sunscreen to protect yourself from UV rays. Seek shade as much as possible to avoid heat exhaustion. Drink water regularly and eat healthily. A short afternoon nap of 15 to 30 minutes could be your best bet to feeling revitalized. And you can get on with your day.

Jen’s a passionate environmentalist and sustainability expert. With a science degree from Babcock University Jen loves applying her research skills to craft editorial that connects with our global changemaker and readership audiences centered around topics including zero waste, sustainability, climate change, and biodiversity.

Elsewhere Jen’s interests include the role that future technology and data have in helping us solve some of the planet’s biggest challenges.

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