Fig. 2

Breastfeeding-induced calming responses in mothers. A 3-min data from the middle of each condition for 23 of 26 participants were used for this analysis. A The schema for the breastfeeding experiment and an example IBI plot. Mothers sat alone for 5 min (resting, grey), sat with her infant on lap for 5 min (BF⁻, light green), breastfed from one side for some time (BF⁺, dark green), and repeated the with infant conditions once more. Vertical arrows indicate the timings when the mother reported the infant’s latch-on and when reported perceiving the milk ejection. IBI and vocalization data in the middle 3 min of these conditions (dashed boxes) were analyzed. B The proportion of the amount of maternal spontaneous vocalization by condition, excluding her replies and reports towards the experimenter. C Proportion of maternal vocalization and its subdivision to the duration of each condition. Each vocalization was categorized by whether it was directed towards the infant or the experimenters, and if the latter, whether it was spontaneous or induced. In B, the sum of “spontaneous to exp” and “to infant” is shown. D Maternal sleepiness scores for each condition, from a post-experiment questionnaire. At the end of the experiment, mothers recalled and chose her level of sleepiness from a range between not sleepy (score: 0) and difficult to stay awake (score: 3) for each condition. E Proportion of infant negative vocalization to the duration of each condition. F–K Heart rate variability measures in the time-domain (F–H) and the frequency-domain (I–K). Bars represent mean ± SEM and dots represent individual data points. Multiple comparisons were done by Friedman’s test, and a post hoc Wilcoxon signed-rank test for all pairs, with p-values adjusted by Benjamini–Hochberg’s method, †p < 0.1, *p < 0.05, **p < 0.01, ***p < 0.001