I've written about this before, but a relatively reliable clock for when I really ought to go to bed is when my body's ability to self-regulate its temperature diminishes and I start getting visibly cold. I'm not sure how or why it happens, but it's one of the things that forces me out of hyperfocus to go "oh yeah, I should reach for a jacket!" at which point I sometimes realize how tired I am, and sometimes obey it.
Right now is one of those times, as I am beginning to shiver. Good night.