In infancy, it’s the extremes of arousal that are ‘sticky’: naturalistic data challenge homeostatic models of self-regulation

Most theoretical models of arousal/regulatory function emphasise the maintenance of homeostasis; consistent with this, most previous research into arousal has concentrated on examining individuals’ recovery following the administration of experimentally administered stressors. Here, we take a different approach: we recorded day-long spontaneous fluctuations in autonomic arousal (indexed via electrocardiogram, heart rate variability and actigraphy) in a cohort of 82 typically developing 12-month-old infants while they were at home and awake. Based on the aforementioned models we hypothesised that extreme high or low arousal states would be more short-lived than intermediate arousal states. Our results suggested that, contrary to this, both low- and high-arousal states were more long-lived than intermediate arousal states. The same pattern was present when the data were viewed over multiple epoch sizes from 1 second to 5 minutes; over 10-15-minute time-scales, high-arousal states were more long-lived than low- and intermediate states. One possible explanation for thesefindings is that extreme arousal states have intrinsically greater hysteresis; another is that, through ‘metastatic’ processes, small initial increases and decreases in arousal can become progressively amplified over time.