Infants show negative changes in affect and physiology when re-experiencing a stressor, its context, and a positive event 24-h later.

Exposure to early life stress shapes further development, affects later stress reactivity, and mental health outcomes. Despite the central role of early experiences, there is little understanding of how these rapidly forgotten events gain their influence. An infant's ability to cope with everyday stressors is founded on successful co-regulation through mother-infant interaction. A significant disruption of this interaction through the Face-to-Face Still-Face paradigm elicits a well-documented behavioral and physiological stress response in infants. What has yet to be explored is whether infants show regulatory adaptions when encountering the situation over again. To fill this gap, 80 mother-infant dyads were observed in the lab on two consecutive days. Infants in the experimental condition (n = 40) were exposed to a double Still-Face paradigm on day one. Infants in the control group (n = 40) completed time-matched episodes of typical play during their first visit. Mother-infant dyads from both groups returned to the lab 24 h later and participated in the double Still-Face paradigm. Changes in behavior (positive and negative affect), physiology (heart rate), and salivary cortisol, compared to day one and between groups, were evaluated and used to infer adaption to the previous experienced laboratory visit. Infants in the experimental condition showed a significant decrease in positive affect (p = 0.016) and an increase in heart rate (p < 0.001) on day two, compared to controls, even during baseline measures and a neutral first play episode. Infants in the control condition showed a significant decrease in affect (p = 0.05) and non-significant increase in heart rate on day two when first encountering the Still-Face paradigm. Infants in the experimental condition showed significant higher heart rate on day two compared to the control group (p = 0.046). Infants in the experimental condition also exhibited a marginally significant increase in salivary cortisol on day two, compared to day one (p = 0.054). The change in infant heart rate was independent of maternal heart rate which did not differ between day one and day two, or between groups. Findings suggest that a previous stressful experience may elicit a behavioral and physiological adaption in infants 24 h later. Our results suggest that even a short, acute stressful event can elicit a lasting stress response in infants 24 h later. The effect we observed was specific to the context of the stressful event, not just the stressor. More precisely, the effect "spilled over" from the stressful experience on day one into the baseline measure of day two, usually a neutral experience. The results could have implications for further research on how stressful experiences may shape the stress response.