I can’t remember what it was called but I remember seeing a documentary about the amount of sleep humans get when artifical light is unavailable, althought fire may be permissable. The effect on brain or physical function was not evaluated. It was found that sleeping hours varied by season and ranged between 10 & 14 hours each night. The patterns of sleep were also interesting; the subjects would wake up after around 7-8 hours for half an hournthen start another cycle. Apparently people felt tired during the day initially but then as the study progressed they felt better than they had previously.