I was rather intrigued/distracted by this the other day.
The hypothesis is: back in the day (up to the industrial revolution), humans used to sleep in two phases. Assuming the sun sets at 8pm and rises at 6am, cavemen would sleep 8pm-midnight, wake up midnight-2am, and sleep 2am-6am. This midnight awake time could be used for reading, praying, sex, or just sitting around.
The Ted talk by Jessa Gamble started this whole thing after a coworker (co-researcher? colleague? these terms all sound so 1800's) sent it to me. (4 min; probably worth watching, for her descriptions of extreme wakefulness in people who tried biphasic sleep)
I went on to read this paper by AR Ekirch in which he gives a lot of evidence for biphasic sleep in premodern times. It's all a bit circumstantial, because they weren't exactly running controlled studies back then. But then a guy named Thomas Wehr ran a study where people were in darkened rooms for 14 hours/day and they started sleeping 8 hours in 2 phases. It got picked up by the NY Times in 1995 and then... forgotten? It's cited (only) 23 times in Google scholar.
Today a search for "biphasic sleep" reveals that it's hit a lot of internet lifestyle-design or primal something or pop-sci blogs, but that they all eventually just cite Wehr and Ekirch. Mary Carskadon doesn't dismiss it, but she hasn't researched it herself. (oh, if you're googling: another term is "segmented sleep.")
Otherwise, it seems to have been lost in history! Mollicone et al 2008 found that split sleep schedules don't positively or negatively affect PVT or subjective sleep scores, but that's about it. Hmm. Gamble is publishing a book about this, but I'm not sure when it's coming out.
My thoughts about this are:
1. I want to study this more!
2. Well, studying the health phenomenon (what happens if you segmented sleep?) is more suited to sleep researchers doing big medical studies. Ugh.
3. However, it'd be sweet if some technology we make could let everyone experiment on themselves.