This is a prospective, randomized-controlled trial that assesses the efficacy of a brief Acceptance and Commitment Therapy (ACT-i), compared to an attentional control group, in adults with chronic insomnia. The interventions will be evaluated for their impact on insomnia severity, cognitive function, depression, anxiety, psychological flexibility, and sleep beliefs - measured before treatment, two weeks after and at a three-month follow-up.

Insomnia is the most prevalent sleep disorder. It is associated with impairments both physically and psychologically. Therefore, it usually causes a suite of durable stressors in numerous aspects of life, such as academically, professionally, socially, and financially. Given its widespread impact, early effective treatments are crucial to prevent long-term consequences.

The first line of treatment is Cognitive-Behavioral Therapy for Insomnia (CBT-i). Despite its effectiveness, approximately 60% of individuals with chronic insomnia and comorbidities do not go into remission after treatment or simply do not adhere to it. Acceptance and Commitment Therapy for Insomnia (ACT-i) has great potential for overcoming this limitation, as it focuses on accepting the feelings and thoughts associated with insomnia, through value-based actions. However, more studies on ACT-i as a monotherapy are needed.

This study aims to evaluate the efficacy of brief, group ACT-i for chronic insomnia in adults, versus an attentional control group. Participants will be aged between 18 and 59 years, diagnosed with chronic insomnia who will be randomly assigned to either ACT-i, or the control group. To limit age-related sleep changes, only participants aged up to 59 will be included, as sleep difficulties tend to increase after the age of 60. The interventions will be performed in groups, in 2 weekly sessions. Their effects will be investigated for the primary outcome related to the severity of insomnia, and secondary outcomes related to depression, anxiety, cognitive functioning, psychological flexibility, and beliefs about sleep at pre-test, post-test at two weeks and three-month follow-up. After the interventions, participants will complete an inventory regarding the adherence and satisfaction to both interventions. This is the first attempt to investigate a group-based ACT-i as monotherapy with an attentional control group delivered in an accessible, brief-group format. Specifically, the sessions are implemented briefly in 2 group sessions, unlike other studies that have (at least) 4 group sessions. Furthermore, the investigators will also compare their adherence rates and patients' satisfaction.

Inclusion Criteria:

• clinical/ subclinical diagnosis of chronic insomnia either already diagnosed by a professional, or identified with SCISD-R by our team of clinicians
• age over 18 and older, but not over 59 years old;
• minimal/ mild symptomatology of depression (scores ≤ 9 on PHQ-9) and/ or anxiety (scores ≤ 9 on GAD-7) or as diagnosed with SCID-5-CV;

Exclusion Criteria:

• diagnosed with a neurological degenerative disorder, or any moderate/ severe psychiatric disorder;
• diagnosed with other sleep disorder (e.g., sleep apnea, restless legs/ periodic limb movements, circadian-based sleep disorder);
• diagnosed with cognitive impairments;
• unable to understand Romanian;
• unable to attend to online-sessions (e.g., no laptop, microphone, camera);

Will share IPD used in the results of the publication, upon request.

The IPD may be shared after publication upon request, with no end date.

The main investigator and statistician of the study will be blinded to the randomization schedule, as a third party will perform the randomization and assign participants to the treatment groups. The online platform will generate the allocation after the inclusion and exclusion criteria are met and the pre-test forms are performed.