It's thought to be a more natural sleep pattern, and us westerners have beaten it out of ourselves:

http://www.bbc.co.uk/news/magazine-16964783

"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. "