Inclusion Criteria:  Presence of insomnia more than three nights per week and for more than three months. Insomnia despite adequate opportunity to sleep. Insomnia severity typical for insomnia disorder, i.e. 11 points or more on the Insomnia Severity Index (ISI). Nighttime insomnia symptoms, i.e., two points or more on at least one of the first three ISI questions. Daytime insomnia symptoms, i.e. two points or more on one or both of the ISI distress and impairment items (numbers 5 and 7). Clinical insomnia symptoms from sleep diaries concerning three nighttime symptoms (difficulties with sleep initiation, difficulties with sleep maintenance, and early morning awakenings), i.e., thirty minutes or more on average on one or more of the symptoms. No current or past CBT-I treatment within the past 5 years. Time and opportunity to participate in treatment for six weeks. Time and opportunity to engage with the treatment content and execute homework assignments for six weeks. Access to a smart mobil telephone, email and internet.  Exclusion Criteria:  Severe depression, i.e., more than 30 points on MADRS-S. Suicidal risk, i.e., 4 points or more on item 9 on MADRS-S. A high intake of alcohol or caffeine, Insomnia due to shiftwork or other sleep-disturbing events (e.g., pregnancy, small children, or animals in the sleep environment). Participants with a history of psychotic or bipolar disorder. If a somatic condition is reported, it is required that it is relatively stable and/or that the candidate is receiving treatment for the condition. When a candidate fulfills criteria for a psychiatric or somatic condition, it is required that insomnia is the disorder currently most distressing and disabling or that the insomnia remains despite treatment for the comorbid condition. Participants with the following primary sleep disorders will be excluded via the Duke Structured Interview for Sleep Disorders (DSISD): sleep apnea, restless legs syndrome, periodic limb movement disorder, circadian rhythm disorder, and parasomnias. If sleep medication is used, it is required that the use has been relatively stable during three months. If Selective Serotonin Reuptake Inhibitors (SSRI) use is reported, it is required that the onset of the medication was at least three months prior to the telephone interview. Participants who regularly consume sleep-disturbing medications.