Ok, this is not just me. You spend a whole day lying out at the beach, and then you get home and you feel exhausted. Why?! I was just laying around! It's because your body was working overtime in three different ways. First, it was protecting itself from UV rays. The sun gives off ultraviolet radiation, and that damages your cells. To repair that damage, your body mounts and immune response and that process increases blood flow to your skin, which also decreases your core body temperature. And that leads us to number two: thermoregulation. You need to keep your body in homeostasis. Its hot at the beach but your body needs to be at 37 degrees Celsius in order to do all the things it needs to do. One way to maintain that temperature is sweating. And when you sweat, your metabolic rate increases and you love water, which leads to more strain and point number three: dehydration. Lower water levels can make blood harder to circulate. So your heart is actually working harder and beating faster to bring oxygen to your organs, and that makes your tired. So cover up, cool off, and drink water!