The sun is messing around with the chemicals in your body. I believe this is why does the sun make you tired. It’s no surprise that the sun rays can penetrate your skin layer and damage it (sunburn, etc.). These chemical effects might produce exhaustion among other after being under the sun for a little too long. Well, these usually happen under a the hottest hour, usually just past noon. Seek shade and it should no be a problem. Drink more water and eat salty snacks, you lose salt and fluid when you sweat. Direct strong sunlight isn’t really enjoyed by your skin. While being under the sun might have its benefits, like the vitamin D, you can’t get overdosed by it, rather your body will refuse to process any additional vitamins. Then from then on you just risk getting sunburned. Just leave the sun if you start to feel uncomfortable if you do insist then make sure you have water readily available around.