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A day at the beach can be exhausting. After lying out in the sun for  hours, you might be thinking, “I was not even moving.

Why am I so tired?” So what is it about soaking up  the sun that tuckers us out? It turns out that when you’re out in the  sun, your body is working overtime to protect itself from heat,  dehydration, and UV damage, making you feel more tired than you did before. Keeping your internal body temperature  constant, or thermoregulating, takes a lot of work!

Your body is pretty good at doing things  to keep your temperature in check, which can wear you out. But it usually doesn’t do much more than  make you want to take a midday snooze. In more serious cases, if your body  overheats too much, you could end up with heat exhaustion, which can lead to a  rapid pulse, heavy sweating, and nausea.

And if your body reaches 40 degrees  Celsius, you could get heatstroke, which can cause fainting, behavior  changes, and sometimes death. So when you’re too hot because you’ve  been sitting in front of a giant ball of plasma for a while, you  start to sweat to cool down. It’s a pretty good solution.

But sweating can also increase your  metabolic rate and lead to dehydration because you’re releasing water from  inside your body to the skin’s surface. While you can usually treat dehydration  fairly easily by drinking more water, in more serious cases it can be a  significant health risk in its own right. When you have less water in your body,  your blood volume decreases a bit, and your heart starts  pumping faster to compensate.

So high temperatures can increase  strain on your heart and lungs. And when you’re out in the sun, your body  isn’t just fighting high temperatures and water loss, it’s also repairing  damage from ultraviolet rays. The sun has different ranges of UV  wavelengths that can harm your cells by promoting mutations and  suppressing the immune response.

UV rays can cut the DNA in your  skin cells or substitute bases, like substituting cytosine for thymine, in  what’s known as a fingerprint mutation. These specific DNA changes are responsible  for more than half of all skin cancers. However, this probably won’t  happen after one day at the beach.

But repeated unprotected sun exposure  can be a cause of these cancers. So, you need to get rid of those affected cells. And to do that, your immune system  increases blood flow to your skin’s surface, which brings white blood cells closer  so they can remove damaged skin cells.

This is why a sunburn turns your skin  red and makes it feel hot to the touch. If your sunburn is itchy, that’s the  feeling of immune cells doing their work. And moving warm blood away from your core also helps reduce your internal temperature.

So the UV repair that your body performs  is also helpful for thermoregulation! Ultimately, when your immune  system is working hard, you feel tired because your body needs rest  so it can efficiently fix the damage you have exposed it to - from heat  to water loss to the sun’s rays. And hearing all of that it is no wonder  being out in the sun can be so tiring!

But the good news is that, unless there  are further complications, this can all be solved by going somewhere cool and  shaded and replacing the water you lost. If you need to take a break from  the heat, watching SciShow videos is an excellent indoor activity. So if you’ve enjoyed this  episode, you might enjoy becoming a member of our channel.

Channel members can submit  questions to our QQ inbox, or see exclusive behind-the-scenes photos. You can get started by clicking  the “Join” button below! [♪ OUTRO].