[burning_hamster]

Some animals get most if not all of their sleep through microsleep [1]. So whatever mechanism "refreshes" the brain, it can work on short time scales. The switch in brain firing dynamics from wake to sleep (NREM) is very fast -- on the order of one to a few seconds. People in team Nedergaard argue that it is the rhythmic neuronal activity during sleep that promotes fluid flow (though to be fair, some argue its arterial pressure). So their answer to your question would be yes, that should be enough time to enhance fluid flow (fluid is flowing all the time, the question debated by scientists is, whether is it being enhanced during sleep).

[1] https://www.science.org/doi/10.1126/science.adh0771

[elric]

I don't think that translates to humans. Sleep disturbances like the micro-arousals triggered by sleep apnea absolutely ruin lives, way before lack od oxygen becomes a problem. Arousals destroy sleep architecture. Humans cannot thrive on microsleep.

[HPsquared]

I wonder how much of an effect physical brain size has on this. Square-cube law and all that. Bird brains are famously small (though highly efficient).