Fig. 1.

Independent contributors to the complex dynamics of human activity, depicted at the top of the figure, include: ➀ reaction to extrinsic random events, ➁ scheduled activities and, ➂ intrinsic factors, notably the endogenous circadian pacemaker which influences the sleep/wake cycle. Our findings of scale-invariant activity patterns (Figs. 2–7) indicate a heretofore-unidentified intrinsic multi-scale control of human activity ➃, which is independent of other extrinsic and intrinsic factors such as ➀, [➁, and ➂. The second panel illustrates an actual one-week recording of human activity [12] during the daily routine protocol. Data structure highlights a 24-h sleep/wake periodic change in the mean activity—lowest during sleep (filled bars). The third panel, expanding a 16-h section of wakefulness, also shows patches of high and low average activity levels with apparent erratic fluctuations at various time scales. The bottom left panel is an activity recording from the same subject during the constant routine protocol with much lower average activity values compared to daily routine. The clear 2-h cycle is a result of scheduled laboratory events. The bottom right panel shows activity levels in the same subject during the forced desynchrony protocol, characterized by a 28-h sleep/wake cycle (as opposed to the 24-h rhythm in activity data during the daily routine).