Persistent somatic symptoms significantly impair the daily lives of individuals with Somatic Symptom Disorder (SSD). Negative psychological factors (NPFs), such as catastrophizing and negative affectivity, further compound this impairment. Yet, few studies have examined strategies in daily life that directly target somatic symptoms and their dynamic relationship with NPFs. The goal of this clinical trial is to investigate if an Ecological Momentary Intervention (EMI) using music listening and cognitive reappraisal can reduce the intensity of and impairment due to somatic symptoms in 40 individuals with Somatic Symptom Disorder (SSD), aged 18-65 years.

Participants will report symptom intensity, impairment, and NPFs up to four times daily via a smartphone app over a six-week period. Using an intraindividual randomized controlled design, during weeks two to five, participants will be randomly assigned in the ratio of 50:25:25 to no intervention, music listening, and cognitive reappraisal, both at fixed times (evening measurement) and during episodes of acute somatic impairment.

Somatic symptoms, particularly when medically unexplained, are common stress-related health issues that significantly affect the daily lives of those experiencing them, especially in the more severe form known as Somatic Symptom Disorder (SSD). Specific negative psychological factors (NPFs), such as catastrophizing, negative affectivity, and rumination about somatic symptoms, may contribute to the intensity of and impairment caused by somatic symptoms (Mewes et al., 2022). The intensity and impairment associated with somatic symptoms are crucial outcome variables for both clinicians and those affected, and should therefore be considered in intervention studies (Rief et al., 2017). The best psychological treatment for somatic symptoms is cognitive behavioral therapy with low to moderate effect sizes (van Dessel et al., 2014). Considering the long waiting time for a therapy appointment and the small effect size, other interventions should be considered. Ecological Momentary Interventions (EMIs; Heron & Smyth, 2010; Mewes, 2022) provide a promising approach for delivering interventions in real time, directly in people's daily lives and their natural environment. By collecting real-time data, EMIs can provide insights into the dynamic relationship between NPFs and somatic symptoms that would otherwise go unnoticed. Because they can address specific events as they occur, EMIs also enable just-in-time interventions in daily life. In addition, real-time assessment of treatment outcomes can contribute to the evaluation of effective interventions. Music listening could be such an intervention. Music listening plays an important role in pain management (Frei & Szucs, 2025) as listening to music in accordance with the individual music preference has an analgesic effect on pain (Van der Valk Bouman et al., 2024). Music that is perceived as happy predicts the reduction of intensity and impairment caused by somatic symptoms in people with SSD (Feneberg et al., 2021). Thus, music listening seems to be a promising low-threshold intervention in daily life. Another promising approach to reduce somatic symptoms is cognitive reappraisal, since people with SSD show significantly less use of cognitive reappraisal than healthy individuals (Schnabel et al., 2022). When cognitive reappraisal is used as a standalone intervention, it reduces symptom annoyance in individuals with medically unexplained somatic symptoms (Kleinstäuber et al., 2019). Furthermore, when combined with other Cognitive Behavioral Therapy interventions such as stress management, it leads to an improvement in the impairment caused by somatic symptoms (Allen et al., 2006; Visser & Bouman, 2001). The present study will examine the effects of an EMI in individuals with SSD, employing an intraindividual randomized controlled design with repeated measures. The investigators hypothesize that, from baseline (week 1) to post-intervention (week 6), participants will show a reduction in the weekly average intensity of somatic symptoms, a reduction in impairment due to somatic symptoms, and a reduction in the strength of negative psychological factors (intermediate effect). Furthermore, the investigators hypothesize that, following an acute event, impairment due to somatic symptoms, the intensity of somatic symptoms, and the severity of negative psychological factors will decrease more during an intervention event than during a control event (immediate effects). In addition, the investigators expect a decrease in somatic symptom severity as measured by the Patient Health Questionnaire-15, a decrease in symptom-related distress as measured by the Somatic Symptom Disorder - B Criteria Scale, and a change in perceived stress as measured by the Perceived Stress Scale -10 from pre-intervention (before week 1) to post-intervention (after week 6). Finally, the investigators hypothesize that higher expectations regarding future somatic symptom intensity will be associated with higher somatic symptom intensity at the subsequent assessment, and that higher expectations regarding future somatic symptom impairment will be associated with higher somatic symptom intensity at the subsequent assessment (fixed measurement points throughout the day). Potential participants will be recruited through flyers, online announcements, and potentially newspaper advertisements. The participants will be German-speaking individuals with SSD. Participants will be screened for SSD using a semi-structured interview based on the DSM-5, an operationalization used in a previous study (Mewes et al., 2022). The sample size was calculated using G*Power (Faul et al., 2007) for an F-test in a repeated-measures ANOVA with the within factors. The main outcome is a change in somatic symptoms, with an estimated effect size of f = 0.35 based on Kleinstäuber (2011), and an assumed correlation among repeated measures of 0.3. With a desired power of 0.90, an alpha error of 0.05, one group, two measurement points (average impairment levels baseline vs. post-intervention), and a nonsphericity correction of 1, the required total sample size was 33 participants. To ensure robustness and improve generalizability, and considering a dropout rate of 20%, the aim is to include 40 participants. Potential study participants will either undergo an initial online screening, during which they will be informed about the study and then contacted by us via phone, or proceed directly to a phone screening. During this call, information about the study will be provided, and a semi-structured interview to check for SSD will be conducted (Mewes et al., 2022). Upon receiving verbal consent to participate in the study, an appointment for the distribution of study materials will be arranged. If participants give their verbal consent during the phone screening, they can complete questionnaires, including potential control variables, before the distribution appointment. A link to the pre-questionnaire on SoSci Survey (Leiner, 2024) will be sent via email. If consent is not given during the phone screening, these questionnaires will be completed at the distribution appointment after consent is obtained. At the distribution appointment, the study information will first be discussed with the participants, and written consent to participate in the study will be obtained. Additionally, study materials (either a study smartphone or the installation of an app on the participants' smartphones) and a study manual will be distributed. The EMI begins one day after material distribution. After the post phase of the EMI, participants will again complete questionnaires on SoSci Survey (Leiner, 2024) regarding somatic symptoms. A return date for the materials will be scheduled via email. At the return appointment after the study period, all study materials will be collected. To achieve the research objective, an intraindividual randomized controlled trial will be conducted. An EMI will be conducted, which will be administered using the software MovisensXS. The study consists of three phases spanning a total of 42 days: The baseline phase (7 days), the intervention phase (28 days), and the post phase (7 days). During all three phases, data will be collected three times a day (11 am, 4 pm, and a self-initiated evening assessment). Additionally, in the intervention phase, an additional measurement point occurs 20 minutes after the evening assessment. During all three phases, participants receive prompts through the app at 11 am and 4 pm. The evening assessment can be initiated before going to bed by pressing a button in the app; participants will also receive a reminder at 9 pm. Additionally, participants will be asked to make self-initiated event-based inputs by pressing a button in the app whenever they feel acute somatic symptoms. They will then receive event-related questions immediately and 20 minutes later via a prompt. During the intervention phase, participants will be randomly assigned each day to either an intervention event or a control event. The intervention takes place in the evening and after self-initiated measurements, which are carried out due to impairment caused by somatic symptoms. On any given day, participants will have a 50% chance of receiving no intervention, and a 25% chance of receiving either music listening (Intervention 1) or cognitive reappraisal (Intervention 2). On days allocated to the intervention event, after the evening measurement, participants will engage in a fixed exercise session and receive another prompt 20 minutes after. In the control event, there will be no exercise between the measurement time points. In addition, at the event-based measurements, participants will again be randomly assigned in a 50:25:25 ratio to either continue with their current activity (control condition), engage in music listening (Intervention 1), or perform a cognitive reappraisal task of their somatic symptoms (Intervention 2).

Inclusion Criteria:

• Somatic Symptom Disorder

Exclusion Criteria:

• chronic physical illnesses that fully explain the somatic symptoms
• current psychotic disorder
• current bipolar disorder
• current self-harming behavior
• current suicidality
• current psychotherapy
• alcohol, drug, or medication dependence within the past 6 months