Being in the sun is hard work or so your brain and body seem to think.

Even when youre just relaxing on a lawn chair by the pool or at the beach, you may have noticed you feel extra sleepy after being outside in the sun for hours. Why does this happen?

The sun causes several changes that result in our bodies needing to work harder, which leads to fatigue, Dr. Heather D. Rogers , a board-certified dermatologist and founder of Doctor Rogers Skin Care, told HuffPost.

We talked to doctors about how the sun affects our bodies and why it can make us feel drained and tips to avoid fatigue and spend time outside safely.

Your body is working hard to regulate its temperature.

Even if [you] are in a lying position by the pool ... your body is constantly working to decrease its temperature to a normal level ... when youre out in the heat, explained Dr. Patrick Mularoni , pediatric ER physician and medical director of sports medicine at Johns Hopkins All Childrens Hospital.

There are three things your body is doing to try to cool itself down: Youre breathing faster, your heart rate is increasing and youre sweating more, he explained. These all require extra energy from your body, which can leave you feeling fatigued.

In extreme cases when our bodies cannot effectively cool down, people can develop hyperthermia or heat-related injury and illnesses, said Dr. Shengyi Mao , an internal medicine physician at The Ohio State University Wexner Medical Center.

Heat exhaustion includes symptoms such as fatigue, dizziness, nausea and headache, according to Mayo Clinic . If heat exhaustion isnt treated, it can lead to heatstroke , which is the most serious heat-related illness and requires emergency medical care.

Your body is trying to repair UV damage.

Too much of the suns ultraviolet rays is very stressful on our skin, Mao explained. When were exposed to ultraviolet light, our bodies are having to fight off and repair cellular damage, which can result in feeling tired not only right away but also afterwards.

Whats exactly happening when sun radiation is damaging skin cells? This causes mutations in the DNA and must be corrected for the cells not to die, Rogers explained.

The redness of a sunburn comes from blood vessels in that area becoming dilated to bring oxygen and nutrients to damaged skin to help with the repair process, she said. The peeling represents dead skin cells that are too damaged to repair.

Overexposure to UV radiation can also weaken the bodys immune system and the skins ability to protect itself from infections, according to the U.S. Environmental Protection Agency . A suppressed immune system leads to more tiredness.

Your body doesnt have enough water to function properly.

In the sun, we are losing water faster due to the heat and evaporation so we are more prone to getting dehydrated, Mao said. When we are dehydrated, our bodies are working harder to do [their] normal functions, like pumping blood around, which can make us feel tired.

Drinking alcoholic and caffeinated beverages can further throw off your water balance and make you even more dehydrated, Mularoni added. (Additionally, alcohol is a depressant that slows down your brain , causing drowsiness.)

So how do you know if youre dehydrated? Mularoni recommended looking at the color of your urine when you pee. Light yellow means you are drinking enough fluids, and dark yellow or orange can indicate you are dehydrated.

If youre getting to that point where your body is saving all of its water and only sending a little bit of water in the urine, your urine is going to be more concentrated, he said. Thats a telltale sign that youre already behind and you need to be drinking more.

Your body is releasing built-up stress.

When youre hanging out by the pool or the beach, this may give you the opportunity to finally slow down and unwind from stress. Your body may recognize that its behind on sleep and can finally recover and relax, which can make you feel sleepy, Mularoni said.