“…However, these methods are inherently limited when the time series' period and amplitude of oscillation, not to mention the wave-shape, are non-constant. Modern time-frequency analysis tools [27], particularly nonlineartype time-frequency analysis tools [58] like the synchrosqueezing transform (SST), have been considered for non-seasonal signals [12]; however, as we will carefully discuss in the supplementary material, both the model behind the original SST and its generalization [34] fail when modulations in frequency, amplitude, or wave-shape do not occur gradually (a phenomenon commonly seen when the underlying human system assumes pathophysiological status). Since well-behaved human systems are rarely of clinical interest, this challenge necessitates a different model for analyzing oscillatory physiological time series and, in particular, a tool for extracting the dynamics encoded by the time-varying morphology of their cycles.…”