I learned years ago not to play Sudoku right before bed. It’s a repetitive thing, and if I get that machine in my head going, and there is nothing else to distract it, the machine just keeps going and going and going; making up imaginary Sudoku patterns and solving them.
Last night’s mistake was to do a round of Spanish on Duolingo right before bed. Got the machine going again, lying there making up imaginary situations that would accommodate my limited knowledge of Spanish, then coming up with words and phrases that would address them.
Sleep would have been more appropriate.