Fibromyalgia: a common problem that requires treatment Fibromyalgia (FM) is a highly prevalent pain syndrome characterized by generalized pain and fatigue, often leading to extensive somatic and psychological distress. The condition is associated with significant suffering and low functional capacity, and also entails high societal costs, as those affected have high levels of healthcare consumption, sick leave, and unemployment. The cause of FM is still unclear and there is no effective treatment. The most evaluated treatment is traditional cognitive behavioral therapy (CBT), which, however, appears to have only minor effects on FM. Exposure-based treatment promising Internet-based exposure-based CBT (Exp-CBT) has previously been evaluated for fibromyalgia in two randomized controlled studies. In the first study, participants (N=140) were randomised to 10 weeks of Exp-CBT or a waiting list. The results showed that iExp had a strong effect on central FM symptoms, pain, fatigue, function and pain-related anxiety. The study also showed that iExp, compared to no treatment, is cost-effective, and that reduced FM-related avoidance behaviors drove the treatment effect. In the second study, participants (N=274) were randomized to Exp-CBT or internet-based traditional CBT (T-CBT). The results showed no significant difference between the groups after treatment. Both groups showed a marked improvement, with an average of 60% (Exp-CBT) and 59% (T-CBT) achieving a reliable change in their symptoms. People with fibromyalgia who receive exposure-based CBT thus appear to experience, on average, a reduction in symptoms and increased function. To date, studies investigating exposure-based treatment for chronic pain have used self-assessment scales to measure the effect of treatment, which is reasonable since pain is a subjective experience. However, one limitation of this is that the investigators therefore have no objective data on whether the aforementioned treatment can also lead to changes in various aspects of pain regulation, i.e., that the brain changes the way it interprets and processes pain. Objective measurement of pain regulation The present study is a longitudinal within-group study, in which the investigators intend to objectively measure various aspects of pain regulation in participants with fibromyalgia (N=45) before and after they have received internet-based exposure-based CBT. Participants undergo pain testing 10 weeks before (T0), at the start of treatment (T1), and at the end of treatment (T2), in order to measure the effect of treatment on participants' pain thresholds, pain tolerance, descending pain inhibitory function, and pain upregulation. Significant potential benefits for patients, healthcare, and society In this study, the investigators want to investigate whether the investigators can objectively see if different aspects of pain regulation are affected in participants who receive the aforementioned treatment. The study is a so-called mechanistic study, i.e., a study to understand more about how our treatment achieves its effect. Such a study makes it possible to be more precise and likely to make the treatment even more effective. The study would also contribute important knowledge to better understand the pain mechanisms in fibromyalgia, a condition with high prevalence and considered very difficult to treat.