How flexible is an individual’s accent during adulthood, and how does this flexibility relate to longer-term change? Previous work has found that accents are remarkably flexible in conversational interaction, but predominantly stable over years, leading to very different views of the role of individuals in community-level sound change. This article examines medium-term accent dynamics (days to months) by taking advantage of a ‘natural experiment’: a reality television show where contestants live in an isolated house for three months and are constantly recorded, forming a closed system where it is possible to both determine the dynamics of contestants’ speech from day to day and reason about the sources of any observed changes. We build statistical models to examine time dependence in five phonetic variables within individuals, in 14.5 hours of spontaneous speech from twelve English-speaking contestants. We find that time dependence in pronunciation is ubiquitous over the medium term: large daily fluctuations in pronunciation are the norm, while longer-term change over weeks to months occurs in a minority of cases. These patterns mirror the conflicting findings of previous work and suggest a possible bridge between the two. We argue that time dependence in phonetic variables is influenced by contrast between sounds, as well as systematic differences between speakers in how malleable their accents are over time; however, we find only limited evidence for convergence in individuals’ accents. Our results have implications for theories of the role of individuals in sound change, and suggest that medium-term pronunciation dynamics are a fruitful direction for future work.