STUDY OBJECTIVES: Heart rate variability (HRV) measures provide valuable insights into physiology; however, gaps remain in understanding circadian patterns in heart rate dynamics. We aimed to explore day-night differences in heart rate dynamics in patients with chronic cardiopulmonary disease compared with healthy controls. METHODS: Using 24-hour heart rate data from patients with chronic obstructive pulmonary disease (COPD) and/or heart failure (n = 16) and healthy adult controls (older group: >/=50 years, n = 42; younger group: 20-49 years, n = 136), we compared day-night differences in conventional time and frequency domain HRV indices and a multiscale-entropy-based complexity index (CI(1-20)) of HRV among the 3 groups. RESULTS: Twenty-four-hour HRV showed significant day-night differences (marked with " big up tri, open") among younger healthy (mean age: 34.5 years), older healthy (mean age: 61.6 years), and cardiopulmonary patients (mean age: 68.4 years), including change in percentage of adjacent intervals that differ > 50 ms ( big up tri, openpNN50), high frequency ( big up tri, openHF), normalized low frequency ( big up tri, opennLF), ratio ( big up tri, openLF/HF), and big up tri, openCI(1-20). Among these, big up tri, openLF/HF (2.13 +/- 2.35 vs 1.1 +/- 2.47 vs -0.35 +/- 1.25; P < .001) and big up tri, openCI(1-20) (0.15 +/- 0.24 vs 0.02 +/- 0.28 vs -0.21 +/- 0.27; P < .001) were significant in each pairwise comparison following analysis of variance tests. Average CI(1-20) was highest in younger healthy individuals and lowest in cardiopulmonary patients (1.37 +/- 0.12 vs 1.01 +/- 0.27; P < .001). Younger healthy patients showed a heart rate complexity dipping pattern (night < day), older healthy patients showed nondipping, and cardiopulmonary patients showed reverse dipping (night > day). CONCLUSIONS: As measures of 24-hour variability, traditional and complexity-based metrics of HRV exhibit large day-night differences in healthy individuals; these differences are blunted, or even reversed, in individuals with cardiopulmonary pathology. Measures of diurnal dynamics may be useful indices of reduced adaptive capacity in patients with cardiopulmonary conditions. CITATION: Ma Y, Chang M-C, Litrownik D, Wayne PM, Yeh GY. Day-night patterns in heart rate variability and complexity: differences with age and cardiopulmonary disease. J Clin Sleep Med. 2023;19(5):873-882.