Here is an interesting article about a research study that received lots of media attention. The researchers studied the sleeping patterns of three traditional societies in Africa and South America - apparently their lifestyles resemble ancient hunter-gatherers. Results showed that the three groups slept 6 hours and 25 minutes per night on average. The sleep duration was similar between the three groups. Despite getting only 6'25" of sleep per night, the participants did not complain of feeling sleepy in the daytime and very rarely took naps.
A surprising result was that natural light did not affect the timing of sleep, as all three groups fell asleep on average 3.3 hours after sunset - which is the time when the temperature was falling. Even more interesting was that all of the group members woke up when the temperature was at its lowest.
Of note, insomnia was extremely rare - two of the groups did not even have a word for it!