system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What are "second winds," really?; or, why do I go from exhausted to wired without sleeping or caffeinating?
gpt
I am not a cell biologist, so this really will be "ELI5" and not "ELI a college student".

Your body has more than one source of energy. Some types are "easy to use, around all the time" that we use in day to day life. Some types are "harder to use, around only when you need it", that we use only when doing strenuous exercise. So when you start exercising, you start using the easy to use energy. After that's run out, you start using the hard to use energy. However, the switch over is not immediate, so you get quite tired between the two, but when the second type kicks in, you feel the "second wind".