BACKGROUND: Financial strain is a well-documented stressor that negatively affects mental health, yet the mechanisms underlying this relationship remain insufficiently understood. This study examines the roles of sleep problems as both mediators and moderators, utilizing the frameworks of stress proliferation and amplification as outlined in the stress process model. METHODS: Data from four waves of the UK Household Longitudinal Study (UKHLS)-Wave 4 (2012-2013), Wave 7 (2015-2016), Wave 10 (2018-2019), and Wave 13 (2021-2022)-were used, covering 34,156 individuals and 103,589 person-years. Fixed effects regression models were employed to analyze changes within individuals over time. RESULTS: Financial strain was associated with deteriorating mental health. Sleep problems-including poor sleep quality, frequent use of sleep medication, sleep disturbance, daytime dysfunction, and longer sleep latency-were also associated with poorer mental health. Importantly, these sleep problems not only mediated the impact of financial strain on mental health but also exacerbated its negative effects. LIMITATIONS: The study could not eliminate the possibility of reverse causality, where deteriorating mental health may influence financial strain or worsen sleep problems. CONCLUSIONS: These findings highlight the importance of addressing financial strain as a key driver of poor mental health. Interventions aimed at improving sleep health may simultaneously mitigate the harmful effects of financial strain, offering dual benefits for mental health. By reducing financial strain and promoting healthier sleep, public health strategies can bolster resilience in populations vulnerable to both financial and psychological stress.