Heart rate (HR) was recorded in healthy full-term newborns aged 1-90 days. The aim of this study was to study the existence of circadian and/or ultradian rhythms in HR to determine maturity. HR was recorded during 24 h, at 30-min intervals, at different postnatal ages. Six-groups were investigated: day 1 (group 1); day 7 (group 7); day 15 (group 15); day 30 (group 30); day 60 (group 60), and day 90 (group 90). The chronograms for HR showed peaks and nadirs along the 24-hour periods, and the cosinor analysis proved the existence of 3-hour ultradian rhythm in groups 1, 7 and 30, and a 12-hour ultradian rhythm in group 90 (p < 0.01 in all cases). The same type of analysis confirmed the existence of a circadian rhythm in group 30. Similar results were obtained for groups 60 and 90 (p < 0.05).
In conclusion: at birth, newborns have an endogenous ultradian period of 3 h. A circadian rhythm appears within 15-30 days of postnatal life.