Excessive sedentariness can impair workers' health and productivity. The move to working from home as a result of the Covid-19 pandemic eliminated many workday opportunities for physical activity. This, coupled with a blurring of boundaries between work and non-work periods, put many at risk of overwork and musculoskeletal issues. We examined how the sudden transition to working from home influenced people's ability to take physically active work breaks. We found that the absence of social norms associated with the presence of colleagues in the work environment left workers uncertain about whether and when it is appropriate to take breaks. The pressure to demonstrate productivity while working asynchronously led to increased sedentariness and decreased break-taking. We propose that online tools that promote flexible social norms around break-taking could empower remote workers to incorporate regular physical activity into their days, without compromising the beneficial aspects of asynchronous working.