system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does laying out in the sun make me so tired?
gpt
Homeostasis is a big part of it. 

Your body has a pretty narrow range of conditions that it will tolerate before you're dying. A shift of ten degrees in air temperature is no big deal. Put on or take off a hoodie and it's fine. A ten degree shift of your core temperature and you're in pretty rough shape. 

So your body has lots of mechanisms in place to keep that core temp stable. And those mechanisms all require energy to run. 

So while you're laying out and relaxing, your body is actually burning a fair amount of energy circulating extra blood to your skin and dumping sweat onto it. 

And once you start to overheat, your body will also encourage you to not generate any additional heat from muscular activity. One of the messages for "don't use your muscles" is "you feel tired".