Fractal regulation of human motor activity and its manifestation at circadian and ultradian time scales

Human activity exhibits a fractal behaviour, characterised by scale-invariant patterns over time scales ranging from minutes to 24 hours. This suggests the existence of a control mechanism with feedback interactions. Aging and Alzheimer’s disease, both marked by an alteration of the suprachiasmatic nucleus (SCN), the circadian pacemaker, have been associated with a reduced scale-invariant correlation. However, the impact of this reduction on ultradian or circadian activity dynamics and its comparison with in-lab circadian measures, remains unclear. Our results link fractal indices, measured with actigraphy, to in-lab circadian markers. Interestingly, fractal scaling in the circadian regime is associated with both daytime rest, interpreted as a fragmentation of sustained daytime activity, and shorter ultradian daytime activity fragmentation. These results also highlight the link between LIDS oscillations at night and scale-invariance at ultradian time scales. Overall, our analysis suggests that previously reported indices of activity dynamics occurring at various time scales might be associated with a common underlying regulation mechanism, involving the SCN.