Higher loneliness is associated with greater positive and negative emotion instability in everyday life.

Loneliness is linked to a wide range of negative outcomes, including worse emotional well-being. Although prior research has demonstrated the relationship between loneliness and of negative and positive emotions, the degree to which loneliness is associated with emotional , a potential indicator of unhealthy patterns of emotional experiences, is unknown. We evaluated whether individual differences in loneliness were related to instability across days in positive emotions (PE) and negative emotions (NE) in daily life. A diverse community sample of 252 adults (age 25-65) completed a baseline assessment of loneliness followed by 14 days of ecological momentary assessments, during which participants reported PE and NE five times each day. Loneliness was significantly associated with greater instability in both PE and NE, after adjusting for demographic characteristics, objective social isolation, and person-mean emotion levels. Notably, the association with PE instability remained significant even after controlling for depressive symptoms, whereas the association with NE instability was attenuated and no longer significant. These findings suggest that loneliness is characterised not only by differences in average emotional states but also by greater emotional variability, particularly in PE, which may be a key feature linking loneliness to broader health and well-being outcomes.