Goal: Everyday life is characterized by various fluctuations in affective wellbeing, stress and episodes of vulnerability, which all might impact mental and physical health. However, these vulnerable states are often subtle and not always directly accessible to the individual (Brosschot et al., 2018). Hypotheses/research questions /objectives: This project aims to develop and validate a real-time system to identify (conscious and subconscious) psychological states associated with increased vulnerability and stress by analyzing transient changes in heart rate variability (HRV). Approach/methods: By recording an ambulatory electrocardiogram (ECG) we aim to use HRV as a sensitive tool to potentially identify these psychological states. Precisely, by applying an ecological momentary assessment (EMA) approach we aim to automatically detect movement-unrelated transient changes in HRV in daily life as triggers for psychologically relevant prompts via the smartphone. In a first step, algorithms sensitive to HRV decreases shall be evaluated in a simulation study using a large and diverse study sample. Second, the functioning and validity of the algorithms will be evaluated in everyday life in order to analyze their generalizability and applicability in the field. Level of originality / innovation: Interactive psychophysiological assessment opens new windows to examine psychophysiological associations as they unfold in daily life with higher precision. By the development of this methodology, an everyday life quasi-experimental approach becomes available, which will allow to translate classical phenomena (e.g., HRV decrease during stressful events) from the laboratory to ecologically valid everyday life contexts. In health and clinical contexts, online detection of additional HRV in real life might serve as a powerful tool to identify situations that pose individuals at risk and could thus be combined with eHealth and mHealth interventions.