system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does the heat make people feel so drained of energy?

I wanted to hit the gym today but when I tried starting I felt so exhausted because of how hot it had been today.
gpt
Well one of the reasons is because, your body is trying to control it's temperature. When you’re exposed to direct heat while spending time in the sun, your body ends up working overtime to keep you cool. Your heart rate and metabolic rate can increase, too. All this extra physiological effort can make you feel tired or sleepy, even if you’ve just been sitting in the sun. 

The second reason is, you’re Dehydrated, even if you’re not engaged in strenuous physical activity, spending time in the sun can quickly dehydrate you. This is partly because your body is working hard to stay cool and partly because you may be losing fluids and salts through sweating. Fatigue is one of the primary symptoms of dehydration, which can increase your risk of heat exhaustion and finally, the third reason is, your body is experiencing chemical changes. The sun’s ultraviolet rays can penetrate the skin and cause damage. The cascade of chemical changes that produce these effects can also cause fatigue after hours in the sun.