Introduction Work is an important part of life, contributing to both health and wellbeing for many employees. However, in recent years, stress-related disorders have received more attention, with an increasing prevalence within the working population. For example, every fourth employee within the European union has experienced stress during most of their working day. Long-term exposure to stressors, such as job strain or interpersonal conflicts, without sufficient recovery, can lead to a dysregulation in the allostatic system, "allostatic load", which constitutes the fundamental features in the development of chronic stress or stress-related disorders. The major diagnostic systems, Diagnostic and Statistical Manual of Mental Disorders (DSM-5) and International Statistical Classification of Diseases and Related Health Problems (ICD-10 and 11) includes sections regarding stress-related disorders. However, the DSM and ICD systems lack an established terminology and criteria for stress induced fatigue and exhaustion. Consequently, the diagnosis "Exhaustion disorder" (ED) was introduced in the Swedish version of the ICD-10 in 2005. ED is manifested by symptoms of extensive mental and physical fatigue, lack of initiative and endurance and prolonged recovery after mental or physical effort. Later international publications have suggested that ED is not an exclusive Swedish condition. Few studies have been published regarding the prevalence of ED, but in a recent study, based on physician-based diagnosis in 3406 participants, 4.2% reported ED.

In addition to stress-related disorders and well-known health implications, such as coronary artery disease, lowered immune functioning, anxiety, depression and insomnia, chronic stress have been associated with impaired work functioning and problems in work participation such as sickness absence and long-term sick leave. In Sweden, for example, stress-related disorders accounted for one fifth of all sick leave during 2019. Decreased work participation is problematic as it has direct effects on people's well-being and leads to immense costs for society. For instance, the total estimated annual costs for work-related stress observed in seventeen OECD countries is considerable, ranging from US $221 million to $187 billion. Given these rising costs, it is not surprising that many policy makers view stress as a major public health issue and are seeking advice on the types of interventions that may be effective.

During the last decades, psychological interventions have been developed in order to increase the individual's psychological resources and resilience to stress. Evidence suggest that stress management interventions are effective in reducing stress in the working population. These results apply to controlled studies targeting general and milder forms of stress-related ill health. However, considering clinical samples, interventions has been less successful, with small effect sizes.

Traditionally, psychological treatments for stress and common mental disorders have not explicitly focused on work-related aspects, such as reducing sickness absence (SA). Recent evidence suggests that psychological interventions are slightly more effective than treatment-as-usual in reducing sickness absence (small effect sizes). However, it is still uncertain what moderates these effects. There are some indications that work directed interventions in combination with psychological treatment are effective with respect to return-to-work (RTW) for those absent with common mental health problems. In a quasi-experimental study by Lagervelt and colleagues comparing CBT to work-focused CBT, full RTW occurred 65 days earlier and partial RTW 12 days earlier in the work-focused CBT group. A significant decrease in mental health problems was equally present in both conditions. These results suggest that by integrating work-related aspects early into the treatment, problems with sickness absence and long-term sick leave can be reduced.

Despite the well-documented efficacy of stress management interventions, and some promising results of work-focused interventions, the range of interventions are not proportionate to the needs among distressed employees. This clarifies the need to further develop and evaluate work directed interventions that are accessible to the working population.

Studies suggest that stress can be managed through internet- and computer-based interventions. In a meta-analysis including 26 controlled studies (n=4226), small to moderate effects were found on outcomes on stress, depression and anxiety. Subgroup analyses revealed that guided interventions were more effective. Studies also suggest that internet-based interventions for stress can have sustained effects on stress reduction, be cost-effective, and have positive effects on participant's experiences on health and wellbeing in both work and private life. However, previous iCBT studies have focused on individuals with elevated stress and few studies has evaluated the efficacy of iCBT in clinical samples, such as employees with stress-related disorder. In addition, we found no previous studies evaluating the efficacy of internet-based and work-focused interventions on sickness absence and RTW.

Purpose of the present study The aim of the current study was to evaluate the efficacy of iCBT for stress-related disorders integrating work-related aspects (W-iCBT) and a generic iCBT, compared to a waitlist control condition (WLC). We hypothesized that W-iCBT and iCBT would be superior and equally effective in reducing perceived stress, burnout, exhaustion, and improving recovery from work and quality of life, compared to a WLC. In secondary explorative analyses, we examined whether the W-iCBT would differ from the iCBT and WLC with regard to important work-related outcomes including work experience, work ability, sickness absence and long-term-sick-leave. We also hypothesized that initially achieved changes would remain stable at a 12-month follow-up.

Method

Design This study was a three-armed controlled superiority trial in which two internet-based interventions for stress-related disorder: (a) cognitive-behavioural therapy (iCBT) and (b) work-focused iCBT (W-iCBT) that integrated work aspects early into the treatment, were compared against a wait-list control group. The study followed the CONSORT guidelines. Sample size was estimated on the basis of previous controlled trials on iCBT for chronic stress, with the aim of detecting an medium effect size on the primary outcome Shirom-Melamed Burnout Questionnaire (see Measures), at post assessment, based on a power of 0.80 in a two-tailed test with 0.05 significant level. Self-reported outcome assessments were collected at pre- and post-treatment (ten weeks) and at a 6- and 12-month follow-up. Participants who met the study criteria and provided informed consent were randomly allocated by an independent researcher using an internet-based random generator (www.randomizer.org). The independent researcher received a list of anonymous identification numbers of all participants and coaches. This procedure ensured that blinding was implemented during randomization. All participants, coaches and those participants reporting benefits due to long-term sick leave at baseline, were randomized in a 1:1:1 proportion. The Ethical Committee of Linkoping University, Sweden, approved all procedures involved in the study (reference number 2016/11-31).

Procedure The study was conducted in a university setting, with researchers and the treatment platform hosted by the university. Participants were recruited from the general public through advertisement, articles in regional and national newspapers, and labour organization magazines. Detailed information and application to the study was presented on the project's homepage (www.istress.se). After initial registration using personal e-mail address, potential participants received an ID number and were asked to (i) provide written informed consent, (ii) complete online screening questionnaires (see Measures) and (iii) participate in a diagnostic interview over the telephone. Following the interview, the included participants were randomized. Participants of the iCBT and W-iCBT groups received access to the programs immediately after randomization and participants of the wait-list control group received access to iCBT/W-iCBT program after the six months follow-up.

Support Every week, participants in both interventions (W-iCBT and iCBT) received personalized written messages from a coach with feedback on the exercises. For the participants in the W-iCBT group, guidance was given on the CBT and work-focused modules simultaneously. The coaches, eight in total, were master level psychology students who were specifically trained to perform feedback according to a standardized manual. The aim of the feedback was to provide support and encouragement and to monitor homework assignments and adherence to the intervention. Treatment as usual was not prohibited but also not encouraged during the trial. The coaches were requested to minimize their support to one message and 15 min of correspondence per week and participant.

Statistical analysis All analysis followed the Consolidated Standards of Reporting Trials (CONSORT) statement for randomized controlled trials. Statistical analyses were conducted following the intention-to-treat principle (ITT) using SPSS version 26. We used the multiple imputation (MI) procedure to impute missing sum scores for participants who did not complete the post- or six months follow-up assessments. MI is considered a to be a conservative approach for analysing incomplete datasets as it takes into account the uncertainty due to missing information. We used all available data from the pre-, post and six-months follow-up assessment as well as age, gender and educational level as predictors. Means, standard deviations, and standard errors of effect sizes were pooled from five sets of imputations. The effects of group on primary and secondary outcome measures of the ITT and completers-only datasets were analysed using repeated measures ANOVAs with time (pre, post and six-months follow-up) as a within-subject factor. Pooled F-values were calculated using RStudio. Cohen's d was reported for the between-group effect sizes and corresponding 95% confidence intervals (95% CI). Analysis of outcomes at baseline and demographical variables between complete and missing data were analysed using T- and Chi2-tests. Completers data on sickness absence and long-term sick leave were analysed using Kruskal-Wallis-test, non-parametric test recommended for comparison of three or more samples. To evaluate clinically significant change, we used the guidelines by Jacobson and Truax (1991). Clinically significant change was based on the completer's analysis. To meet the criteria for clinically significant change on the primary outcome SMBQ, participants had to demonstrate a reliable change of 0.69 and score under the cut-off of 4.4, in accordance with a recent study in a clinical sample by van de Leur et al. (2020). We completed the clinically significant change analysis with KEDS. On the KEDS, participants had to demonstrate a reliable change of 8.72 and score under the cut off of 19.