Does brushing your teeth immediately after sleeping have some medical significance?

Context:

I have generally only brushed my teeth once-a-day, before going to bed.

My dentist has advised to start brushing twice a day, adding "immediately in the morning" to my dental routine.

I don't eat breakfast in the morning, and told the dentist that, and they confirmed that first thing in the morning was still the recommendation.

I didn't press them more, but now wish that I had done so.

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I would have assumed that (generally speaking) brushing at 12-hourly intervals would be the more beneficial than brushing twice, 8 hours apart, and then not brushing for 16 hours.

Since I go to bed at ~11:30pm, therefore if I'm going to brush again, and I don't eat breakfast, then I'm better off brushing around 11:00am, or just before lunch.

I can see 2 possibilities for why that might not be the case:

• Physiological: Sleep does something to the mouth, that means it's beneficial to clean it immediately after waking.

• Psychological: In practice, people are more successful at adding steps to a morning routine, so if they're told to do it 1st-thing, then they're more likely to actually do it. Hence, if I think I can reliably do it in the late morning, then I should do that, instead.

The latter seems very likely to be true.

But is the former option also true?

Is there any medical significance to brushing your teeth after waking up, if you're not going to eat until lunch?