What happens at night? Differentiating within-day and overnight affective inertia

Affective inertia - the degree to which affect persists over time - has, for example, been linked with neuroticism, depressive symptoms, and increased distress. The typical statistical approaches modelling affective inertia as autoregression largely ignore that assessment periods covering several days also include overnight intervals which may bias affective inertia. In this study, we thus aimed to test (1) whether affective inertia differs within-day and overnight and (2) whether within-day and overnight inertia are differentially associated with psychological functioning (e.g. personality, perseverative thoughts, stress). We operationalised within-day inertia as the autoregression of affect from one timepoint to the next during the same day and overnight inertia as the autoregression of affect from the previous night to the next morning. We used data from the ESCAPE project including 254 ethnically and economically diverse participants (25-65 years) who participated in up to three 14-day measurement bursts with five daily beeps. We found significant within-day and overnight affective inertia in positive and negative affect. Overnight inertia substantially exceeded within-day inertia that would be expected for the longer overnight interval, indicating affective inertia differs within-day and overnight. This research highlights the importance to disentangle within-day and overnight intervals when studying affective inertia.