For my PhD I aim to use a comprehensive and multi-method approach, encompassing diverse assessment tools and covering a relatively under-explored developmental period from birth to 5 months of age. By incorporating neural measures such as EEG and fNIRS, along with behavioural observations and parent-reported questionnaires, this study aims to capture a holistic view of regulation and reactivity during the neonatal and early infancy periods. This multi-dimensional assessment strategy aims to examine markers of regulation and reactivity prior to prolonged exposure to external postnatal influences, thereby shedding light on their inherent nature and developmental origins.