system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: you need sleep and food to regain energy, what kind of energy is this and where in the body is it stored?
gpt
Well, energy from food is stored short term as sugar in your blood (your blood sugar levels, the thing diabetics tract.) and long term as fat across your body.

“Energy” from sleep is different. It is not the same thing as the physical calories we need as fuel. We aren’t 100% sure why we need sleep yet, but the theory is that our Brain just needs a break. Like a muscle that’s been working out all day or a computer that’s been running for a week straight, things just stop working correctly. Nerves/synapses don’t fire right, signals dont travel all the way they’re supposed to, etc etc. and that the rest our brain gets while we were asleep is essential for it to reset itself and be able to function the best. 

Again, we don’t exactly know what the physical/chemical mechanisms are that make us feel sleepy/re-energized before and after sleep, but it is quite clear when looking at sleep deprived individuals that the brain just starts to not function as well.