system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does rest/inactivity often make us more tired, while activity can make us more energetic?

The world is currently struggling with both an influx of major stress while people in great numbers are also unemployed and inactive. This is leading to a massive spike in depression and related symptoms. For these reasons, I was wondering if someone could give me a good explanation for why resting often makes exhausting situations even more exhausting. Thanks!
gpt
Think about it like percentages. If you have 100 energy and spend 90 of that on daily things you have 10% left. 

Increasing strength makes tasks easier so you might only use up 80 per day. Increasing cardio makes starting energy higher, say 120. Now you have 33% left at the end of the day.

You mention mood so I'll add that exercise is a positive feedback loop. It makes you feel better and more energetic so it's easier to get more exercise so you feel better and more energetic... works the same way in reverse for laziness.

In a sentence, exhaustion is relative to how much energy you have.