Why Does the Sun Make You Tired?


The sun may give you a boost of vitamin D (1) and happiness-promoting hormones (2). But too much sun can also tire you out, even when you're not exerting yourself.

The sun is a powerful source of heat and light that triggers a number of changes in the human body. While many of these changes are positive, they can also leave you feeling sleepy.

You're Fighting to Keep Cool

To keep your body working properly, you need to maintain a core temperature of around 98.6 degrees Fahrenheit (3). Like other mammals, humans regulate their inner temperature using a process called thermoregulation, which compensates for fluctuations in the outside temperature.

One way the body sheds heat in overly warm environments is by sending warm blood to the surface of the skin, where the heat can more easily evaporate. This process can make skin look flushed in the heat. Another way the body thermoregulates is by sweating, which causes you to lose heat as the sweat evaporates. Sweating doesn't work as well in humid conditions (4), because the sweat doesn't evaporate as easily. In contrast, a strong breeze (5) can help you sweat more efficiently.

Being too warm can lead to heat stroke (6), a potentially fatal illness. To prevent you from reaching this point, fatigue (7) sets in when you get too warm. This fatigue should discourage you from persisting with activity that may be harmful if you overdo it.

You're Dehydrated

Sweating causes you to lose water and sodium, which can lead to dehydration (8). Dehydration makes your blood thicker, which interferes with thermoregulation because thickened blood is less able to carry body heat away from your core. The thickened blood is also harder to pump, so it puts extra strain on your heart. With less oxygen (9) being pumped around your body, you will likely feel lethargic.

Common symptoms of dehydration include thirst, dark urine, headaches, dizziness, and dry lips. Dehydration also causes fatigue, and in severe cases, it may lower blood pressure and lead to organ damage.

To keep from getting dehydrated, it's important to drink lots of fluids. The usual recommendation is to drink two to three liters of water a day, and more if you're sweating. Avoid diuretics, such as caffeine and alcohol, and consider choosing drinks with electrolytes (10), such as energy drinks, to keep a healthy balance of water and nutrients (11).

You're Sunburnt

In addition to heat, the sun exposes you to ultraviolet (UV) rays, which can make you feel tired. Too much sun exposure can lead to a sunburn, and chronic exposure to the sun can increase the risk of skin cancer (12).

To protect itself against the sun's UV rays, the skin synthesises melanin, the dark pigment responsible for what is referred to as a tan (13). Since UV rays damage the skin, your immune system also kicks in to try to protect you against sun exposure. The immune system is typically restored during sleep, so increased work by the immune system can make you feel sleepy (14).

You're Exercising

The summer holidays often find people at the beach, on an athletic field, or out hiking. All that exercise can add to the fatigue caused by the sun. Exercise taxes your muscles and drains your energy (15). Studies show that your muscles require even more energy than usual (16) when you exercise in the heat. As your muscles use up your body's energy and nutrients, exercise makes you steadily more tired.

Your Sleep-Wake Cycle Is Shifting

Humans evolved to be awake in daylight (17) and go to sleep at night, but artificial lights and digital device screens have extended normal wake times well into the night. Spending all day in the sun can trigger changes to your hormones that restore the natural day-night cycle. This sun exposure makes you feel sleepy earlier in the evening.

How Can You Stay Alert on Sunny Days?

Maintain your energy in the heat by staying well-hydrated and keeping as cool as possible. Some ways to protect yourself from the sun (18) include:

  • Drink Fluids: Bring a reusable water bottle and set a timer to remind yourself to drink water even before you feel thirsty (19). Favor cold drinks, as these have the added advantage of cooling you down. Fruits like watermelon can also be a good source of fluids. If you're doing vigorous exercise, a sports drink might be a better choice as it replenishes your electrolytes.
  • Eat and Drink Wisely: Try to stay away from caffeine, alcohol, sugar, and excessive salt, as these can contribute to dehydration.
  • Avoid the Hottest Parts of the Day: The sun shines strongest in the late morning and early afternoon, so these are good times to stay indoors or seek shade.
  • Take Breaks: You should also take breaks from sun exposure throughout the day, especially if you're exercising. If you can't find a cool, shady spot, look for an air-conditioned (20) public library or shopping mall.
  • Protect Yourself: Loose clothes made of breathable fabrics and light colors are best at keeping you cool. Don't forget to bring sunglasses and a hat. Also, put on sunscreen and reapply it throughout the day.
  • Get Enough Sleep at Night: Sleep deprivation (21) can leave you more vulnerable to heat-related illnesses, so aim for a solid night's sleep if you plan on doing strenuous activity in the sun. Optimizing your bedroom with breathable bedding and a fan or air conditioning can help you sleep better in warm weather.

Children, older people, those with health conditions, people taking certain medications, and individuals who are sensitive to heat should take extra care on hot, sunny days. It's a good idea to familiarize yourself with the signs of heat illness so you can prevent them from becoming worse. If you notice the early signs of heat illness, move to a cool place out of the sun, drink fluids, and seek medical attention if necessary.


+ 21 Sources
  1. 1. Accessed on March 21, 2021.https://pubmed.ncbi.nlm.nih.gov/20072137/
  2. 2. Accessed on March 21, 2021.https://pubmed.ncbi.nlm.nih.gov/12480364/
  3. 3. Accessed on March 21, 2021.https://pubmed.ncbi.nlm.nih.gov/29939615/
  4. 4. Accessed on March 21, 2021.https://pubmed.ncbi.nlm.nih.gov/18461221/
  5. 5. Accessed on March 21, 2021.https://pubmed.ncbi.nlm.nih.gov/28985477/
  6. 6. Accessed on March 21, 2021.https://medlineplus.gov/heatillness.html
  7. 7. Accessed on March 21, 2021.https://pubmed.ncbi.nlm.nih.gov/21029187/
  8. 8. Accessed on March 21, 2021.https://pubmed.ncbi.nlm.nih.gov/32310416/
  9. 9. Accessed on March 21, 2021.https://pubmed.ncbi.nlm.nih.gov/20689090/
  10. 10. Accessed on March 21, 2021.https://pubmed.ncbi.nlm.nih.gov/26702122/
  11. 11. Accessed on March 22, 2021.https://medlineplus.gov/fluidandelectrolytebalance.html
  12. 12. Accessed on March 21, 2021.https://pubmed.ncbi.nlm.nih.gov/16600340/
  13. 13. Accessed on March 21, 2021.https://www.fda.gov/radiation-emitting-products/tanning/risks-tanning
  14. 14. Accessed on March 21, 2021.https://pubmed.ncbi.nlm.nih.gov/30920354/
  15. 15. Accessed on March 21, 2021.https://pubmed.ncbi.nlm.nih.gov/19402743/
  16. 16. Accessed on March 21, 2021.https://pubmed.ncbi.nlm.nih.gov/25943663/
  17. 17. Accessed on March 21, 2021.https://pubmed.ncbi.nlm.nih.gov/31534436/
  18. 18. Accessed on March 21, 2021.https://medlineplus.gov/ency/article/000056.htm
  19. 19. Accessed on March 21, 2021.https://medlineplus.gov/ency/patientinstructions/000865.htm
  20. 20. Accessed on March 21, 2021.https://www.cdc.gov/disasters/extremeheat/faq.html
  21. 21. Accessed on March 21, 2021.https://pubmed.ncbi.nlm.nih.gov/15912679/

Related Reading:

  • How Long is an Ideal Nap?

    Naps can have benefits for adults across the lifespan. Discover the ideal nap length and the best time of day to nap, depending on your unique circumstances.

  • How Much Do Dogs Sleep?

    How much sleep do dogs really need? We explore the research explaining why dogs sleep so much and provide tips to help them get adequate rest.

  • Do Fish Sleep?

    Do fish sleep? Fish do exhibit sleep-like behavior, but it can look quite different from humans and other animals. Learn more about how and where fish sleep.