system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: what exactly happens to your brain when you feel mentally exhausted?

Is there any effective way to replenish your mental energies other than sleeping?
gpt
Your brain cells (neurons), just like any other cells in your body, excrete metabolic wastes. During the day (or night) when you are active and awake, the brain is slightly swollen relative to when you are sleeping and this shrinks the amount of space between the gyri and sulci (mountains and valleys) of the brain. The less space, the less efficiently the cerebrospinal fluid is circulated and cleaned. With this stagnating CSF, the metabolites being released by brain cells just hang around the cells in the brain and this affects the metabolic functions of said cells. All of this occurs until we go to sleep, where the brain shrinks and more room is made for the CSF to be circulated and the metabolites to be cleared.