Resource depletion leads to apathy and burnout. Because you need quality rest to build up your resources and kickstart that cycle: rest — tackle tasks with enthusiasm — rest again in time. And repeat the process. Often, we don't notice this «resource drain» in time and keep going as if there's still fuel in the tank. But in reality, it's empty. And the car isn't moving. But you're pushing it. You're sweaty and muttering curses. Not fun at all. But you keep pushing.
And now it seems like, «Well, duh, obviously: you're tired — get some rest! It's common sense!». But in reality, we always stumble into this feeling «Just a little more, I'll push through a bit longer, and then I'll rest for everything and everyone at once». For me, this state is like being in debt: your resource balance is zero, but you keep borrowing against it, sinking further into the red, instead of stopping, being kind to yourself, and giving yourself some time to recover.