Background: Insomnia is the most prevalent sleep disorder worldwide, and regularly co-occurs with anxiety and depression. Cognitive behavioural therapy is the gold standard treatment for insomnia (CBT-I), however demand for treatment providers drastically exceeds supply. Internet-delivered programs for insomnia (iCBT-I) improve treatment access. However the effects of unguided iCBT-I for individuals with comorbidities within a naturalistic setting remains unexplored. We developed a novel unguided iCBT-I program and evaluated its impact on insomnia, psychological distress, and wellbeing when accessed by the public. Methods: 317 participants experiencing insomnia for over 3 months enrolled in the program. The program consisted of 4 lessons delivered online with automated web support. Insomnia symptoms, psychological distress, and general wellbeing were assessed at lesson 1 and 4. Intention-to-treat linear mixed models were used to examine effects on insomnia, distress, and wellbeing. Results: Participants experienced large (g = 1.11) and significant reductions in insomnia, moderate (g = 0.55) and significant reductions in distress, and small (g = 0.37) but significant improvements in wellbeing. 65% of participants who reported pre-treatment insomnia severity at clinical levels remitted following treatment. Limitations: To examine the program in a naturalistic setting, we did not employ a control group or follow participants beyond the completion of treatment. Conclusions: Unguided iCBT-I is effective for individuals in the community who experience insomnia and are likely experiencing comorbid mental health problems. These effects in the absence of guided contact strengthen the utility of unguided iCBT-I as a scalable and cost-effective method of disseminating treatments for this disorder.