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Occurrences of heartbeats (•), as a function of the respiratory phase (0° for initiation of inhalation; 180° for beginning of exhalation), as taken from a typical subject during deep sleep. The heart-rate trend (magenta plot) shows respiratory sinus arrhythmia (RSA)—accelerated heartbeat during inhalation and retarded heartbeat during exhalation. The intensity of the RSA is determined by the amplitude of sinus modulation.
