Figure 7. Sleep timing across the lifespan.

(a) Sleep timing as measured by mid-sleep on days with no social constraints, (b) sleep duration, and (c) difference between wake-time and the minimum of the circadian wake propensity across the lifespan. Data for sleep timing and duration³⁴,⁷⁰ are shown as black circles. Results are shown for: day/evening light levels of 700/40 lux (purple); day/evening light levels of 700/20 and 700/10 lux (green); the light profile in Fig. 2a for the Hadza (blue)²². Further details on the age dependence of the circadian amplitude and the rise of the sleep homeostat during wake are given in Supplementary Material. The intrinsic circadian period is set to 24.2 h.