Non-linear cardiac dynamics and morning dip: an unsound circadian rhythm

The frequency of sudden cardiac death increases in the morning. The relationship between decreased complexity of heart rate dynamics and sudden cardiac death has been documented. An understanding of circadian variation in the complexity of cardiac dynamics may be important to predict and prevent sudden cardiac death. Dynamic 24‐h electrocardiographic recordings were obtained from 30 healthy ambulant subjects aged 41–50 years, and the digitized data were partitioned into sections of 30 min duration. For each section, four indexes obtained from separate algorithms of non‐linear dynamics of the RR interval – modified correlation dimension, Lyapunov exponent, approximate entropy, and fractal dimension – were calculated. Normalized low‐ (0·04–0·15 hertz) and high‐frequency (>0·15 hertz) components were also calculated. All four indexes of non‐linear dynamics showed a remarkably similar circadian rhythm: a prominent morning dip preceded by a steep decline during the late night, a recovery during the evening and a peak around midnight. In the morning, the low‐frequency component rose rapidly with concomitant reduction in the high‐frequency component. The complexity of cardiac dynamics decreases significantly in the morning, and this may contribute to the ominously increased rate of cardiac death in the morning hours.