Purpose <p>This secondary analysis of a randomized clinical trial aimed to understand the cost-effectiveness of cognitive behavioral therapy for insomnia (CBT-I) in improving absenteeism (i.e., time away from work) and presenteeism (i.e., unproductivity while at work) among cancer survivors.</p> Methods <p>A total of 55 currently employed mixed cancer survivors who met DSM-5 criteria for insomnia disorder and self-reported cognitive impairments were randomized to receive seven weekly, individual CBT-I sessions immediately or after a waiting period. Participants completed the Work Productivity and Activity Impairment Questionnaire (WPAI). Information from participants and the Labour Force Survey (LFS) were used to calculate costs. Education-adjusted mixed-effects models using intention-to-treat principles assessed immediate and longer-term effects of treatment on work productivity.</p> Results <p>While CBT-I was not associated with significant improvements in absenteeism, the treatment group reported a 23.5-point reduction in presenteeism post-treatment, compared to a 0.45-point decrease in the waitlist control group. Improvements in presenteeism were maintained at 6-month follow-up. The mean cost of total work productivity loss was CAD627.59 per person per week before beginning CBT-I. Treatment resulted in a 48.4%, 44.6%, and 30.5% reduction in lost productivity immediately, 3 and 6 months post-treatment, respectively. Total cost savings for the first year after treatment, adjusting for treatment costs, were estimated at CAD 9478.82.</p> Conclusions <p>Intervening upon late and long-term effects of cancer treatment (e.g., sleep, fatigue, cognitive impairment) through CBT-I produces meaningful and durable improvements in work productivity, particularly presenteeism.</p> Implications for Cancer Survivors <p>With appropriate treatment, survivors can address side effects and increase productivity, but additional work is needed to improve access to and coverage for evidence-based interventions.</p>