Aerobic Training-Induced Changes in Sedentary Time, Non-Exercise Physical Activity, and Sleep Among Breast Cancer Survivors and Postmenopausal Women Without Cancer

Background/Objectives: The 24 h activity cycle highlights the need to consider sedentary behavior (SED), non-exercise physical activity (NEPA), and sleep when introducing aerobic exercise. This study assessed changes in these components among breast cancer survivors (BCS) and non-oncologic menopausal women after a 3-month walking (W) or Nordic walking (NW) program. Methods: A total of 324 menopausal women participated: 156 non-oncologic (Meno), 102 BCS with natural menopause (BCS_Meno), and 66 BCS with medically induced menopause (BCS_Ind_Meno). Linear Mixed Effects (LME) modeling was applied. Age, BMI, hormonal therapy, cancer treatments, hypertension, sleep, and METs were included as covariates. Results: BCS_Meno and BCS_Ind_Meno had longer sleep durations at baseline (adj. diff.: +26.5 min/day, 95% CI 10.1 to 43.0; p = 0.002 and +25.7, 95% CI 6.7 to 44.6; p = 0.008). Sleep improved across all groups post-intervention (overall adj. effect = +17.4 min/day, 95% CI 4.8 to 30.0; p = 0.007). Higher sleep and METs were associated with reduced SED (sleep: β = −43.7 min/day per unit increase, −52.6 to −34.8; METs: β = −115.4, −126.4 to −104.4; both p < 0.001). A significant group × time interaction showed a decrease in SED in the BCS_Ind_Meno group (adj. diff. = −65.1 min/day, −102.8 to −27.4; p = 0.001). NEPA was negatively influenced by sleep (β = −8.7 min/day, −16.2 to −1.1, p = 0.024) and positively by METs (β = +121.1, 111.8 to 130.3; p < 0.001). NEPA increased only in BCS_Ind_Meno (adj. diff.: +70.6 min/day, 38.4 to 102.7; p < 0.001), not in BCS_Meno (+9.87, −18.7 to 38.4; p = 0.497). Conclusions: BCS_Ind_Meno showed the greatest benefits, with reduced SED, increased NEPA, and improved sleep. Sleep improved across all groups following aerobic interventions.