I'm more of a low carb person rather than keto, but I can anecdotally confirm that how many carbs and/or sugar I have the day before does impact my alertness the next day regardless of sleep.

I also do time restricted feeding and skip breakfast. If I load up on carbs (particularly crappy ones) the day before, I feel hunger much more the next morning than if I ate clean during my feeding window.

The body is a complex machine.