Actually, my problem didn't start with the 1903 update, but with the October 8, 2019—KB4517389 (OS Build 18362.418)—update to 1903, which my system installed on 11 October. It is otherwise as described—nothing fixes it. I’m forced to turn off my display at night. I forgot to specify, but I've got an Nvidia graphics card, for which I reinstalled the latest update—no joy.
I filed a bug with Microsoft, but my bet is that nothing will happen until some future update will mysteriously fix the problem, and no explanation or apology will be forthcoming.