[paul.fr]

Why is having a wash, and brushing your teeth, so refreshing in the morning?

[moonfire]

It awakens your senses!  LOL

[Carolyn]

And makes you smell good.

[another_someone]

In terms of brushing your teeth, amongst other things, it will reduce the pH of your mouth (since toothpaste is alkaline, as is saliva which will also be stimulated by brushing), with the bacteria (and, if you are prone to gastric reflux, then that too), will increase the acidity in your mouth.

In general, you are likely to dry out (mouth and skin) overnight, so washing all will help with that.

The general rubbing that goes with washing will probably help the blood supply a bit.