Anne Skeldon and Derk-Jan Dijk (Surrey Sleep Research Centre) have co-authored a News and Views article for Nature on “Biological Rhythms: human sleep before the industrial era”.  It has appeared in the November issue and can be found here. A quantitative study of sleep patterns in three pre-industrial societies implies that our natural sleep duration is close to seven hours, and that sleep cycles are determined by environmental temperature as well as the light-dark cycle.