Persistent "inability to feel it" w/r/t/ achievement and performance level you used to summon for your work can also be an early sign of depression, which is something you might want to be vigilant and proactive about.

There's a link at the bottom of the article which is a differential analysis of burnout vs. major depression. The biggest difference seems to be that depression affects all facets of one's life, but people suffering from burnout at work can still experience happiness outside of work (e.g., if they take a vacation).

Except it isn't quite accurate. It is certainly possible to experience major depression as a result of burnout. I know because I have experienced it myself. Also, a lot of the things in that differential diagnosis chart aren't correct (e.g. major depression has been shown to affect the immune system and the HPA axis).