system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: When you lose sleep you feel your body being heavy. What happens inside your body when you don't sleep enough ?
gpt
We really don't know much about sleep. Scientists really don't understand the mechanisms by which it works, why it's important, or even its real purpose - which is crazy considering how important it is to our health. 

We know some of the side effects of lacking sleep - your brain cells have a hard time communicating with each other, and your hormone secretion gets out of whack - this is largely what causes the effects you listed in your question.

As for WHY those things happen, we really just don't know. It's really pretty crazy how poorly sleep is understood.