When you’re sick, you don’t want to move. You don’t want to get up. You just want to sleep and rest – so your body can heal itself. Tiredness is a signal that inactivity is desired so that repairs can occur.
Is depression the same thing? Is it the mind’s way of stopping thought, and manufacturing inactivity, so that the brain can heal itself?
This would fit in with the idea that mental disorders like depression are signs of metabolic problems or inflammation/infection.