system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do I get tired when I'm not doing anything?
gpt
i have answered a lot of questions this way today, but I'll explain it in this context.

Your sense of "energized" and "tired" are a direct result of the balance in your ANS (autonomic nervous system), of which there are two branches, sympathetic (stimulatory) and parasympathetic (inhibitory). Usually both are partially active and your level of alertness is the balance between these two.

So, when you are engaging in stimulating activities (reading, running, whatever) your sympathetic nervous system activity increases and overrides your parasympathetic making you feel more alert. Conversely, the exact opposite is true during periods of inactivity.