Do we have any idea what it actually _is_ what is getting depleted when we start to run out of energy or concentration? It's something I've wondered about alrea

Do we have any idea what it actually _is_ what is getting depleted when we start to run out of energy or concentration? It's something I've wondered about already a few times - I mean I can pretty much feel running out of steam myself. Which means there is some way in which my mind is able to measure whatever it is that it causing this.