Figure 2. Timing of sleep and circadian rhythmicity when exposed to natural light/dark cycles.

(a–c) Average light profiles (yellow) and sleep and wake timing (blue) (±standard deviation in red) for the Hadza and the San people who live a traditional hunter-gather lifestyle with no access to electrical light²². Sleep-timing as determined by the mathematical model using these same light profiles (grey) compare well with those determined in the field. (d–f) Computed sleep timing in relation to sunset and sunrise. Timing of the computed minimum of the circadian wake propensity is indicated by the orange circles. No fitting has been carried out for any of the model predictions: intrinsic circadian period is set to the human population average of 24.2 h; parameters for homeostatic rise during sleep and circadian amplitude have been set to be consistent with the age of the participants (see the Supplementary Material for further details on the relationship between parameters and age and the values of all other parameters).