Insomnia is a common sleep problem that impacts both physical and mental health. Acceptance and Commitment Therapy for Insomnia (ACT-I) is proposed as a promising new treatment for insomnia. However, its effectiveness without overlapping behavioral components of the current standard treatment, cognitive behavioral therapy, remains largely unknown. The main goal of this clinical trial is to test the efficacy of stand-alone ACT-I in adults with insomnia, compared to a waitlist control group. The second goal is to explore the potential working mechanisms of ACT-I, using network intervention analysis. Adults with insomnia will be randomized to an ACT-I or waitlist group. Both the ACT-I treatment and waiting period span 7 weeks. Assessments take place at baseline (pre), after 4 weeks (mid), and after 8 weeks (post), followed by a 3- and 6-month follow-up for the ACT-I group. The main questions this RCT aims to answer are: * Is ACT-I superior to a waitlist control group in improving insomnia severity (primary outcome) in addition to sleep diary measures, anxiety, depression, general mental health, and sleep-related quality of life (secondary outcomes)? * How do networks of insomnia symptoms or potential treatment processes (i.e., psychological (in)flexibility, sleep-related arousal, dysfunctional cognitions, and sleep-related safety behaviors) develop during and following ACT-I?