Endocrine therapy is an established treatment for hormone receptor-positive breast cancer, but can cause significant side effects with deterioration in quality of life. The side effects of all forms of endocrine therapy are well recognized and can lead to treatment non-persistence or non-compliance. Compliance is defined as the degree or extent of conformity to the recommended administration by the provider, whereas persistence refers to the act of continuing treatment for a certain prescribed duration. Treatment adherence is especially important in breast cancer, as early cessation or reduced compliance to hormonal therapy are associated with reduced disease-free survival and increased mortality. Chronotherapy, also called chronotherapeutics, is defined as the administration of a medication in coordination with circadian rhythm in order to minimize side effects and yield a greater efficacy. The investigators propose to perform a pragmatic, multi-centre, open-label, randomized clinical trial to establish the optimal timing (morning vs. evening) of administering endocrine therapy based on side effects and benefits in early stage breast cancer patients.
