Including the question I was wondering about: do the authors think the historians who have argued medieval Europeans had segmented sleep were therefore wrong, and humans have never slept like that? The answer seems to be "no", but here the explanation gets to be what sounds, to me at least, plausible but a bit speculative,
[The historian Roger] Ekirch combed through centuries of Western literature and documents to show that Europeans used to sleep in two segments, separated by an hour or two of wakefulness. [Author of the present study Jerome] Siegel doesn’t dispute Ekirch’s analysis; he just thinks that the old two-block pattern was preceded by an even older single-block one. “The two-sleep pattern was probably due to humans migrating so far from the equator that they had long dark periods,” he says. “The long nights caused this pathological sleep pattern and the advent of electric lights and heating restored the primal one.”