Background Office workers spend 70–85% of their time at work sitting. High levels of sitting have been linked to poor physiological and psychological health. Evidence shows the need for fully powered randomised controlled trials, with long-term follow-up, to test the effectiveness of interventions to reduce sitting time. Objective Our objective was to test the clinical effectiveness and cost-effectiveness of the SMART Work & Life intervention, delivered with and without a height-adjustable workstation, compared with usual practice at 12-month follow-up. Design A three-arm cluster randomised controlled trial. Setting Councils in England. Participants Office workers. Intervention SMART Work & Life is a multicomponent intervention that includes behaviour change strategies, delivered by workplace champions. Clusters were randomised to (1) the SMART Work & Life intervention, (2) the SMART Work & Life intervention with a height-adjustable workstation (i.e. SMART Work & Life plus desk) or (3) a control group (i.e. usual practice). Outcome measures were assessed at baseline and at 3 and 12 months. Main outcome measures The primary outcome was device-assessed daily sitting time compared with usual practice at 12 months. Secondary outcomes included sitting, standing, stepping time, physical activity, adiposity, blood pressure, biochemical measures, musculoskeletal issues, psychosocial variables, work-related health, diet and sleep. Cost-effectiveness and process evaluation data were collected. Results A total of 78 clusters (756 participants) were randomised [control, 26 clusters (n = 267); SMART Work & Life only, 27 clusters (n = 249); SMART Work & Life plus desk, 25 clusters (n = 240)]. At 12 months, significant differences between groups were found in daily sitting time, with participants in the SMART Work & Life-only and SMART Work & Life plus desk arms sitting 22.2 minutes per day (97.5% confidence interval –38.8 to –5.7 minutes/day; p = 0.003) and 63.7 minutes per day (97.5% confidence interval –80.0 to –47.4 minutes/day; p < 0.001), respectively, less than the control group. Participants in the SMART Work & Life plus desk arm sat 41.7 minutes per day (95% confidence interval –56.3 to –27.0 minutes/day; p < 0.001) less than participants in the SMART Work & Life-only arm. Sitting time was largely replaced by standing time, and changes in daily behaviour were driven by changes during work hours on workdays. Behaviour changes observed at 12 months were similar to 3 months. At 12 months, small improvements were seen for stress, well-being and vigour in both intervention groups, and for pain in the lower extremity and social norms in the SMART Work & Life plus desk group. Results from the process evaluation supported these findings, with participants reporting feeling more energised, alert, focused and productive. The process evaluation also showed that participants viewed the intervention positively; however, the extent of engagement varied across clusters. The average cost of SMART Work & Life only and SMART Work & Life plus desk was £80.59 and £228.31 per participant, respectively. Within trial, SMART Work & Life only had an incremental cost-effectiveness ratio of £12,091 per quality-adjusted life-year, with SMART Work & Life plus desk being dominated. Over a lifetime, SMART Work & Life only and SMART Work & Life plus desk had incremental cost-effectiveness ratios of £4985 and £13,378 per quality-adjusted life-year, respectively. Limitations The study was carried out in one sector, limiting generalisability. Conclusions The SMART Work & Life intervention, provided with and without a height-adjustable workstation, was successful in changing sitting time. Future work There is a need for longer-term follow-up, as well as follow-up within different organisations. Trial registration Current Controlled Trials ISRCTN11618007. Plain language summary Office workers spend a large proportion of their day sitting. High levels of sitting have been linked to diseases, such as type 2 diabetes, heart disease and some cancers. The SMART Work & Life intervention is designed to reduce office workers’ sitting time inside and outside work. The SMART Work & Life intervention involves organisational, environmental, group and individual strategies to encourage a reduction in sitting time and was designed to be delivered with and without a height-adjustable workstation (which allows the user to switch between sitting and standing while working). To test whether or not the SMART Work & Life intervention worked, we recruited 756 office workers from councils in Leicester/Leicestershire, Greater Manchester and Liverpool, UK. Participants were from 78 office groups. One-third of the participants received the intervention, one-third received the intervention with a height-adjustable workstation and one-third were a control group (and carried on as usual). Workplace champions in each office group were given training and resources to deliver the intervention. Data were collected at the start of the study, with follow-up measurements at 3 and 12 months. We measured sitting time using a small device worn on the thigh and collected data on weight, body fat, blood pressure, blood sugar and cholesterol levels. We asked participants about their health and work and spoke to participants to find out what they thought of the intervention. Our results showed that participants who received the intervention without workstation sat for 22 minutes less per day, and participants who received the intervention with workstation sat for 64 minutes less per day, than participants in the control group. Levels of stress, well-being, vigour (i.e. personal and emotional energy and cognitive liveliness) and pain in the lower extremity appeared to improve in the intervention groups. Participants viewed the intervention positively and reported several benefits, such as feeling more energised, alert, focused and productive; however, the extent to which participants engaged with the intervention varied across groups. Scientific summary Background High levels of sedentary behaviour (e.g. sitting, reclining or lying, and expending ≤ 1.5 metabolic equivalents) have been linked to poor health outcomes, including type 2 diabetes, cardiovascular disease, some cancers and premature mortality. In addition to physiological health outcomes, high levels of sitting are detrimentally associated with cognitive function, mental health and a lower quality of life. Working-age adults spend around 60–70% of their workday sitting, with workdays being more sedentary than non-workdays; however, this can vary by occupation. Office workers spend 70–85% of their time at work sitting and accumulate a large proportion (40–50%) of this time in prolonged sitting bouts. Office workers also typically spend a large proportion of their leisure time sitting, compared with other occupations. In the workplace, lower levels of sitting have been linked to higher work vigour, higher job performance and lower presenteeism. Workplaces are, therefore, an ideal setting for implementing interventions to reduce daily sitting. Current evidence shows a need for fully powered randomised controlled trials (RCTs) with long-term follow-up to test the effectiveness of interventions to reduce sitting. Previous work from our group, evaluating multicomponent interventions to address high levels of sitting in office workers, found that significant reductions in sitting time across the day were mainly driven by changes to workplace sitting and to not daily sitting, indicating that a whole-day approach to encourage reductions in daily sitting was needed to maximise the potential health benefits. Objectives The main aim of the study was to evaluate the clinical effectiveness and cost-effectiveness of the SMART Work & Life (SWAL) intervention (provided with and without a height-adjustable workstation) in a sample of desk-based office workers. If both interventions were shown to be effective in comparison with the control group, then a secondary aim would be to determine if one intervention was more clinically effective and cost-effective than the other. Primary objective To investigate the impact of the SWAL intervention, delivered with and without a height-adjustable workstation, on device-assessed daily sitting time compared with usual practice at 12 months’ follow-up. Secondary objectives To investigate the impact of the SWAL intervention, delivered with and without a height-adjustable workstation, over the short term (assessed at 3 months) and longer term (assessed at 12 months) on: daily sitting time on any valid day (3 months) and on workdays and non-workdays sitting time during work hours daily time spent standing and in light and moderate or vigorous physical activity (MVPA) across any valid day, during work hours and on workdays and non-workdays daily time spent stepping and number of steps across any valid day, during work hours and on workdays and non-workdays markers of adiposity [i.e. body mass index (BMI), per cent body fat, waist circumference] blood pressure blood biomarkers [i.e. fasting glucose, cholesterol, triglycerides, glycated haemoglobin (HbA1c)] musculoskeletal health psychosocial health (i.e. fatigue, stress, anxiety and depression, well-being and quality of life) work-related health and performance (i.e. work engagement, job performance and satisfaction, occupational fatigue, presenteeism, sickness absence) sleep duration and quality. To undertake a full economic analysis of the SWAL intervention. To conduct a mixed-methods process evaluation throughout the intervention implementation period (using qualitative and quantitative measures) with participants and workplace champions. Methods Design A three-arm cluster RCT with a cost-effectiveness and process evaluation analysis. Follow-up measures were taken at 3 and 12 months. Setting Local councils in Leicester, Leicestershire, Greater Manchester and Liverpool, UK. Participants Participants were recruited from across participating councils (i.e. Leicester City Council, Leicestershire County Council, Salford City Council, Bolton Council, Trafford Council and Liverpool City Council). Participants were office-based employees (aged ≥ 18 years) who spent the majority (≥ 50%) of their day sitting, were at least 60% full-time equivalent and were able to walk without assistance. Employees who were pregnant, who already used a height-adjustable workstation or were unable to communicate in English were not eligible. Participants were grouped into clusters either by a shared office space (although could be made up of different teams/departments) or if they were members of the same team but split into different office spaces. To be eligible, each cluster was required to have at least one participant willing to undertake the role of workplace champion and at least four participants in the cluster. Informed consent was obtained from participants before the baseline measurement session and verbal consent was confirmed at each follow-up. Sample size To detect a 60-minute difference in average daily sitting time between the intervention groups and the control group [assuming a sitting time standard deviation (SD) of 90 minutes, 90% power, a two-tailed significance level of 5%, an average cluster size of 10 (range 4–38), an intraclass correlation coefficient of 0.05, the number of clusters being inflated by a factor of 1.23, allowing for one cluster drop out per arm and a 40% loss to follow-up/non-compliance with the activPAL (PAL Technologies Ltd, Glasgow, UK)], the required sample size was 690 participants from 72 clusters. Testing two intervention arms independently with the control arm was also taken account of as part of the sample size calculation. Interventions The SWAL intervention is a multicomponent intervention grounded in several behaviour change theories, which aims to reduce daily sitting in office workers. The SWAL intervention includes organisational-level behaviour change strategies (e.g. management buy-in), environmental-level behaviour change strategies (e.g. relocating waster bins, printers) and group-/individual-level behaviour change strategies (e.g. education, action-planning, goal-setting, addressing barriers, group coaching, challenges, self-monitoring) that are delivered by workplace champions. After all baseline measures were carried out, clusters were randomised to one of the following three conditions: (1) SWAL only, (2) the SWAL intervention with the addition of a height-adjustable workstation (i.e. SWAL plus desk) or (3) the control group. Randomisation was stratified by area (i.e. Leicester, Salford or Liverpool) and cluster size [i.e. small (