system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does spending a few hours out in the sun at the beach make you get so tired?

Or any amount of time out in the sun for that matter, but particularly the beach.
gpt
Dehydration. The beach tends to have a constant breeze, so you don't notice yourself sweating as much as you actually are. 

There also is very little shade, so you body overheats a little. This also means furthering dehydration, but it also throws your body out of equilibrium, exactly like having a fever.