Scale-Invariant Aspects of Cardiac Dynamics Across Sleep Stages and Circadian Phases

We find that key scale-invariant features of heartbeat dynamics, which have been previously associated with the underlying mechanisms of cardiac regulation, change significantly with sleep-wake transition, across sleep stages and circadian phases under both healthy and pathologic conditions. Our findings indicate that sleep-wake and circadian cycles do not simply modulate basic physiologic functions by generating rhythms with a fixed periodicity, but also influence the neural regulation of fundamental physiologic systems such as the cardiovascular system simultaneously over a broad range of time scales. Our empirical observations suggest that the neural systems of sleep and circadian regulation play an important role in the scale-invariant fractal organization of cardiac dynamics (and perhaps also of other physiologic dynamics), which has been shown to breakdown with disease and advanced age.