Active night-time tweeting is associated with meaningfully lower mental wellbeing in a UK birth cohort study.

It has been suggested that use of social media late at night could lead to worse mental health outcomes. We linked Twitter ('X') data to self-reported measures of mental health from the Avon Longitudinal Study of Parents and Children. We aimed to predict these measures from the average hour a participant posted Tweets. We used data from 310 adult participants and 18,288 Tweets. We found strong evidence the average hour participants posted Tweets was associated with depressive symptoms, anxiety symptoms, and mental wellbeing. Average hour explained almost 2% of the variation in mental wellbeing, comparable to reports of the impact of binge drinking and exercise. Participants who, on average, Tweeted through the night (23:00 to 05:00) showed meaningfully worse mental wellbeing than those who Tweeted during the daytime. Although the average hour that a participant posted their Tweets explained less of the variation in their depressive (0.2%) and anxiety (0.7%) symptoms, after stratification by age and sex these relationships became stronger. Our results could inform behavioural interventions for improving the safety of social media platforms.