system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:why does spending time in the sun make me feel tired?

I find that I am often zonked after spending time in the sun....even if I don't do much exercise?  I wonder why?
gpt
If you're not dehydrated (which can happen) or suffering from heat exhaustion (which can also happen), it can be due to temperature regulation. Your body has to stay pretty near 98.6 degrees F. 

When you're too warm or too cold for a long time, your body has to regulate your temperature that whole time and that takes real energy, just like exercise (but unlike exercise your heart doesn't get any real benefits...)