Fall asleep or freeze? In home settings, extreme low and high levels of ambient noise associate with reductions in infants’ spontaneous movement.

Early exposure to noisy, chaotic home environments is associated with increased physiological stress and adverse long-term cognitive and clinical outcomes. However, this research has generally used questionnaires to measure average household noise, despite that stress is, by definition, a dynamic, compensatory mechanism – thus obscuring the detailed picture of how we are dynamically influenced by, and compensate for, stressors in our environment. Here, we used miniaturised microphones and autonomic monitors to measure noise and infants’ spontaneous movement at home. We observed an inverse-U-shaped relationship, such that less movement was observed at extreme low and high ambient noise levels. This relationship was observed across multiple settings and time scales; however, when we removed the autocorrelation (i.e. slow-varying fluctuation) from the sound data, it disappeared and only a linear relationship (higher movement associated with higher sound) was observed. This indicates that the effect of reduced movement at extreme high sound is driven by sustained episodes of high sound. This effect may be caused by temporary fatigue, or by children actively down-regulating their movement levels to compensate for highly arousing situations - analogous to a newborn closing their eyes when over-stimulated. Implications of these findings for understanding how noisy/chaotic environments affect development are discussed.