English used to have a word for pre-dawn regret
Before 'anxiety' was a diagnosis, it was a specific hour — and English had a word only for that hour.
Old English had a word, uhtceare, that meant one very narrow thing: lying awake before dawn, worrying. Not general anxiety, not insomnia — the specific flavor of dread that shows up in the hour before the sky lightens, when your problems seem enormous and your ability to solve them seems small. The word disappeared from the language centuries ago. The hour didn't.
Mark Forsyth built The Horologicon around the idea that English used to be full of hyper-specific words like this, one for nearly every hour and mood of the day, and then quietly dropped most of them. There was fudgelling, for pretending to work. Rizzling, for the drowsiness after a heavy lunch. Uhtceare is his opening act, and it's the best one, because it names a feeling most people assume is un-nameable — too personal, too three-in-the-morning to have a word.
It did have a word. English just let it go, the way a language sheds anything it stops needing to say out loud.
You still get the feeling. You just have to describe it in a sentence now, instead of a word.