Cognitive behavioural therapy for insomnia is efficacious and recommended for insomnia, but availability is scarce. Cognitive behavioural therapy for insomnia self-help interventions could increase availability, especially if unguided. Optimizing cognitive behavioural therapy for insomnia methods and system user-friendliness, we developed a short, digital, self-help programme-FastAsleep-based on the behavioural components of sleep restriction and stimulus control. This study investigated its feasibility and preliminary effects. Thirty media-recruited participants with moderate to severe insomnia were assessed via telephone before using FastAsleep for 4 weeks, and were interviewed afterwards. Self-ratings with web questionnaires were conducted at screening, pre-, mid- and post-treatment, and at 3-month follow-up. Primary outcomes were feasibility (credibility, adherence, system user-friendliness and adverse effects), and secondary outcomes were changes in symptom severity (insomnia, depression and anxiety). Adherence was generally high, participants' feasibility ratings were favourable, and adverse effects matched previously reported levels for cognitive behavioural therapy for insomnia. Symptoms of insomnia decreased after the treatment period (Hedge's g = 1.79, 95% confidence interval = 1.20-2.39), as did symptoms of depression and anxiety. FastAsleep can be considered feasible and promising for alleviating insomnia symptoms among patients fit for self-care. Future controlled trials are needed to establish the efficacy of FastAsleep and its suitability in a stepped care model.