"Ekirch kept finding references to something called 'first sleep' in medieval texts. The term was never defined, suggesting that contemporary readers were already familiar with it. But Ekirch was at a loss.
Meanwhile, Wehr was bringing people into the Maryland woods and putting them to bed at dusk, wondering how the absence of artificial light would affect their biological clocks during long winter nights. As his experiment went on, an unexpected pattern emerged. Subjects were getting the eight hours of sleep long thought to be normal, but it came in two shifts. In between was a period of unusual wakefulness lasting an hour or two. Wehr characterized it as a sort of 'altered consciousness.' Subjects typically reported feeling something like bliss.
Fragmented sleep is typically viewed as a disorder, and as Warren points out, there are now medications designed to provide eight hours of uninterrupted sleep. But as Ekirch realized upon learning of Wehr's experiment, segmented sleep may actually have been the norm for humanity until the invention of the lightbulb.
Warren repeats Wehr's protocol to give his account of this forgotten state of mind, and also delves into the practice of lucid dreaming - in which the dreamer awakens just enough to control the proceedings.
Gradually, his pursuit for neurological 'special effects' morphs into a penetrating inquiry into the nature of consciousness itself."
Update: here's a talk by the author and an interview...