Quantification of irregular circadian cycles using time-series methods

The original focus of the field of chronobiology was to demonstrate that most - if not all - biological parameters are modulated in a regular way by the periodic alternation of day and night, called circadian cycles. Recently, it has become clear that adverse conditions such as ageing and/or disease induce a reduced or more irregular modulation, such that it has become of interest to quantify these irregularities as a proxy to understand underlying pathology. In the present contribution, we explore how recently developed time-series decomposition tecniques such as SSA, EMD, EEMD and CEEMDAN may be applied to chronobiology to describe irregularities in circadian cycles.