Assessment of central and cardiac oscillatory phase–phase coupling during sleep

Previous research on the dynamic interactions between the brain and the heart during sleep has mainly focused on phasic cortical events, such as arousals and K-complexes. However, little is known about spontaneous and continuous brain–heart coupling during sleep. The current study investigated the instantaneous phase–phase synchronization of cardiac cycles with brain electrical activity across sleep stages, without relying on internal or external events as triggers. Electrocardiography (ECG) and electroencephalography (EEG) data from ten neurosurgical patients during different sleep stages were utilized. EEG was recorded from intracranial hippocampal and neocortical electrodes, as well as from scalp electrodes. EEG phase was computed using complex wavelet analysis, and ECG phase was modeled based on the R-peaks. Phase–phase synchronization was quantified within 10-s windows for coupling factors of 1: m, where m (ranging from 1 to 10) is the multiplier for the ECG phase. Surrogate data were employed to ensure that the observed synchronization was physiological rather than random. The findings demonstrate statistically robust EEG–ECG synchronization within the delta, theta and alpha EEG bands across different brain sites. phase–phase coupling was stronger in scalp EEG than in intracranially recorded EEG signals. The predominant EEG coupling frequency was significantly lower during N2 sleep compared to the waking state, indicating sleep-dependent modulation of EEG–ECG phase–phase coupling.