system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 - Why does the sun tire you out?

For example if you just sun tan all afternoon, physically you don’t really do anything but lay there. Then afterwards feel sleepy and tired. What causes this?
gpt
When you sit in the sun, your body is absorbing the energy of the light hitting you as heat.

This means your body heats up.

Our bodies don’t like heating up or cooling down too much, they like to stay at about the same normal body temperature.

So, when you’re in the sun, you body has to do extra things to get rid of that extra heat (assuming it’s not like, a cold day)

This would primarily be through sweating.

So even just sitting in the sun all day, your body is using up energy to dump the extra heat, and it is also losing water through sweating, which adds to the tired/groggy/headache feeling.

This also doesn’t mention the extra energy your body has to spend on repairing your skin from all the damage the sun is causing.