HEART-RATE BIORHYTHM CHANGES DURING THE FIRST 3 MONTHS OF LIFE

Heart rate (HR) was recorded in healthy full-term newborns aged 1-90 d ays. The aim of this study was to study the existence of circadian and /or ultradian rhythms in HR to determine maturity. HR was recorded dur ing 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 ch ronograms for HR showed peaks and nadirs along the 24-hour periods, an d 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 existenc e 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.