Insomnia is a common problem in the general population. To treat insomnia, medication therapies and insomnia-related cognitive-behavioral interventions are often applied. The aim of the present study was to investigate the influence of acceptance and commitment therapy (ACT) on sleep quality, dysfunctional sleep beliefs and attitudes, experiential avoidance, and acceptance of sleep problems in individuals with insomnia, compared to a control condition. A total of 35 participants with diagnosed insomnia (mean age: 41.46 years old; 62.9% females) were randomly assigned to the ACT intervention (weekly group therapy for 60–70 min) or to the active control condition (weekly group meetings for 60–70 min without interventional and psychotherapeutic character). At baseline and after eight weeks (end of the study), and again 12 weeks later at follow-up, participants completed self-rating questionnaires on sleep quality, dysfunctional beliefs and attitudes about sleep, emotion regulation, and experiential avoidance. Furthermore, participants in the intervention condition kept a weekly sleep log for eight consecutive weeks (micro-analysis). Every morning, participants completed the daily sleep log, which consisted of items regarding subjective sleep duration, sleep quality, and the feeling of being restored. Sleep quality, dysfunctional beliefs and attitudes towards sleep, emotion regulation, and experiential avoidance improved over time, but only in the ACT condition compared to the control condition. Improvements remained stable until follow-up. Improvements in experiential avoidance were related to a favorable change in sleep and cognitive-emotional processing. Micro-analyses showed that improvements occurred within the first three weeks of treatment. The pattern of results suggests that ACT appeared to have improved experiential avoidance, which in turn improved both sleep quality and sleep-related cognitive-emotional processes at longer-term in adults with insomnia.