BackgroundThe purpose was to prospectively examine the effects of sedentary behaviors on subjective memory impairment in breast cancer survivors (BCS) and the extent to which sleep disturbances mediated this pathway.MethodsBCS (N = 380; Mage = 57.38 ± 9.25 years) completed questionnaires assessing demographics, health history, sitting behaviors, sleep disturbance, subjective memory impairment, and moderate‐to‐vigorous physical activity (MVPA) at baseline and 6‐month follow‐up. A subsample (N = 300) wore an accelerometer to objectively estimate sedentary time and MVPA. Structural equation modeling was used to test direct and indirect effects of self‐reported and objectively estimated sedentary behaviors on memory impairment (through sleep disturbance) across time. Models were adjusted for demographic, clinical, and MVPA covariates.ResultsAt baseline, more total daily sitting (γ = 0.23), occupational sitting (γ = 0.11), television viewing (γ = 0.15), and computer use (γ = 0.22) were associated with greater sleep disturbance, which was associated with greater memory impairment (γ = −0.22). Indirect effects of self‐reported sitting on memory were significant. At follow‐up, increased total daily sitting (γ = 0.08) and computer use (γ = 0.14) predicted increased sleep disturbance, which predicted increased memory impairment (γ = −0.09). The indirect path from increased computer use to memory impairment was significant (β = −0.01). In the accelerometer subsample, greater daily sedentary time at baseline was associated with less sleep disturbance (γ = −0.14) and memory impairment (indirect effect: β = 0.03).ConclusionsFindings provide early evidence that sedentary contexts may differentially influence sleep disturbance and memory impairment in BCS. Computer use and television viewing may pose the strongest risks to cognitive health. Disparate findings between objective and subjective sedentary measures warrant further research.