The varieties of human experience... Siri Hustvedt is writing about falling asleep, with particular emphasis on the falling (sometimes an actual falling feeling, sometimes shades and colours, or a string of words of more or less random connection with whatever went before); and about insomnia and trying to avoid it. Which is where the varieties come in. She says:
I read in the afternoons now, never at night, because books enliven the internal narrator to one vivid thought after another.
Me, I read first thing in the morning and in the early evening. If I try to read last thing at night, so far from books 'enliven[ing] the internal narrator to one vivid thought after another', that voice is very quickly overcome by a smothering drowsiness and begins to merge with other voices presaging the onset of sleep. No more than a strong cup of coffee just before midnight can reading keep me from going under.