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Background: Breast cancer survivors (BCS) can exhibit a dysregulation of cortisol and elevated C-reactive protein (CRP) levels post-treatment, which increase the risk of diverse health outcomes. Certain behavioural, physical, and psychological variables may help to predict cortisol and CRP levels post-treatment. The aims of this study were to: (1) describe naturally occurring changes in absolute diurnal cortisol and CRP levels over a period of 1.5 years post-treatment among BCS, (2) assess if absolute diurnal cortisol and CRP levels change in tandem, and (3) assess behavioural, physical, and psychological variables as predictors of absolute diurnal cortisol levels and CRP levels.

Methods: Capillary blood and saliva samples were collected from 201 BCS, on average, 3.5 months post-treatment (T1) and again 3, 6, 9, and 12 months later (T2-T5). At each time point, five saliva samples were collected on two nonconsecutive days: at awakening, 30 ?min after awakening, 2:00 p.m., 4:00 p.m., and at bedtime. At each time point, participants also completed self-report questionnaires and wore an accelerometer for seven consecutive days. Data were analyzed using multilevel modeling.

Results: Absolute diurnal cortisol levels did not change significantly over time. CRP levels decreased across time points (B _linear ?= ?-0.31, p ?= ?.01), though the rate of decrease slowed over time (B _quadratic ?= ?0.05, p ?= ?.03). Generally, greater sedentary time predicted higher overall absolute diurnal cortisol levels (B ?< ?0.01, p ?= ?.01); whereas higher physical activity (B ?= ?-0.004, p ?< ?.01), lower body mass index (B ?= ?0.10, p ?< ?.01), and lower health- and cancer-related stress (B ?= ?0.24, p ?= ?.04) predicted lower overall CRP levels. Also, lower absolute diurnal cortisol levels were evident when participants engaged in more sedentary time, as compared to their own average sedentary time (B ?= ?-0.01, p ?< ?.01).

Conclusions: Results offer insight into the nature of change in diurnal cortisol and CRP levels among BCS from treatment completion onwards and offer clinical implications. Helping BCS manage their weight, reduce stress, increase physical activity participation, and decrease sedentary time as soon as possible after treatment may help to reduce physiological dysregulations, thereby lowering the risk of adverse health outcomes in this population. Further research investigating specific intervention parameters such as type, context, frequency, and intensity are warranted for the development of the most optimal interventions.