It seems reasonable that our ancestor's did not sleep in eight hour sessions. It appears, according to historical documents, recent studies, and modern hunter-gathering tribes, that our natural inclination is the sleep in two four-hour shifts, with a couple of waking-hours between them.
Many of us today find ourselves awake in the middle of the night and assume there is something wrong. Perhaps that is not the case. Perhaps we are naturally inclined to wake at night, and perhaps it kept our ancestors alive more often than people who passed out for eight hours in a row.
From the BBC:
In 2001, historian Roger Ekirch of Virginia Tech published a seminal paper, drawn from 16 years of research, revealing a wealth of historical evidence that humans used to sleep in two distinct chunks.His book At Day's Close: Night in Times Past, published four years later, unearths more than 500 references to a segmented sleeping pattern - in diaries, court records, medical books and literature, from Homer's Odyssey to an anthropological account of modern tribes in Nigeria.Much like the experience of Wehr's subjects, these references describe a first sleep which began about two hours after dusk, followed by waking period of one or two hours and then a second sleep."It's not just the number of references - it is the way they refer to it, as if it was common knowledge," Ekirch says.