Fig. 3.

Phase synchronization as a measure of cardiorespiratory coupling across sleep stages. A significant increase in CRPS is observed during deep sleep (DS) and light sleep (LS) compared to REM sleep and quiet wake, indicating a significant modulation in cardiorespiratory coupling due to sleep regulation. Left bars represent the group mean n∶m phase synchronization obtained by averaging the percent of synchronization for each subject in the group. Error bars indicate the standard error. The analysis is based on data from 189 healthy subjects during 8 h of sleep by exploring all n∶m ratios, for n > 1 and m ≤ 3 (where n is the number of heartbeats occurring in m adjacent respiratory cycles). Statistical significance of the results for each sleep stage is demonstrated by a comparison to a surrogate test (right bars) performed on the same group of subjects, where the Fourier phases of the respiratory data from each subject were randomized (36) prior to phase-synchronization analysis. A Mann–Whitney rank sum test comparing the synchronization obtained from the real data to the synchronization of the surrogate data yields p < 10^-3 for each sleep stage. See Materials and Methods for details on the surrogate data and test. Dashed line (not a fitting line) highlights the sleep-stage stratification pattern in CRPS.