As part of a start-up you're expected to be insanely productive at all times - so much so - that "normal" productivity feels like you're spinning your wheels and not getting anything done.
The other side is the company encouraging being a "hero". Being the one who can do anything, who gets X, Y, Z done on time, even if it takes all night. The praise and recognition fades quickly - as you realize you've set the expectation such that you can function at 200% at all times. (And you start feeling stressed as you fall to 150%, to 120%, to 100%.)
Some of the comments in this thread are spot on. For the level where I'm at (not full-bore burnout), I'm trying to prioritize my life outside of work again (aka have a life outside work). Sleep well, exercise, pick up hobbies again, go for hikes again, and so on.
Burning out to me - is letting work take up a disproportionate amount of your time and mental energy - so much so that the microcosm of work starts feeling like the most important thing.
Getting out of it (for me) is all about regaining that perspective.