BACKGROUND:Retirement is a life transition involving an obligatory change in how people use their time. Because there are strong associations between use of time and health, different changes in time use following retirement may have different impacts on mental health. METHODS:105 participants were followed from 6 months before retirement to 12 months after retirement. At each time-point, use of time was quantified using a validated computerised 24-hour recall. Depression, anxiety and stress were assessed using the Depression, Anxiety and Stress Scales (DASS21), well-being with the Short Warwick-Edinburgh Mental Well-being Scale (SWEMWBS), life satisfaction with the Australian Unity Personal Well-being Index (AUPWI), and self-esteem with the Rosenberg Self-Esteem Scale. Time-use data were analysed using compositional data analysis, which treats the 24-h day as a holistic "activity composition" rather than as individual activity domains. Time flow analytics were used to map patterns of change in time use from pre-retirement to post-retirement. Regression analysis was used to determine whether changes in the activity composition were significantly associated with changes in mental health. Compositional isotemporal substitution models were used to illustrate dose-response relationships between changes in time use and conditional changes in mental health for individual activity domains, such as sleep, screen time and physical activity. RESULTS:Following retirement, time no longer spent in work flowed mainly to household chores, sleep, screen time and quiet time (e.g. reading). Mental health improved overall. Changes in the activity composition were significantly related to conditional changes in DASS21 total score, depression, stress, and self-esteem, but not to anxiety, well-being or life satisfaction. Replacing work time with physical activity or sleep was associated with positive changes in mental health. Effect sizes for 60-minute substitutions ranged from -0.15 to +0.31. CONCLUSION:Following retirement, replacing work with physical activity, and to a lesser extent sleep, is associated with better mental health.