Background and Objectives: Alterations in the habitual sleep/wake patterns of women with breast cancer and their informal caregivers may be concurrently exacerbated and co-vary during the patient’s treatment. The current study set out to longitudinally ex-plore sleep-wake patterns of patient-caregiver dyads in the context of adjuvant chemotherapy (CTh) for breast cancer. Taking into consideration the complexity of mechanisms interfering with a care dyad’s sleep, diverse sleep-impairing factors were also investigated. Design and Methods: Descriptive, observational, repeated-measures dyadic study. Forty eight newly diagnosed women receiving outpatient adjuvant CTh for early stage breast cancer (stage I-IIIA) and their nominated primary informal caregiver completed self-reported sleep measures at pre-treatment (week prior to CTh), post-CTh cycle 1, post-CThC4, and approximately 30 days after the end of CTh (total of =6 cycles received). Additional data on the dyads’ sleep hygiene practices (SH), patient physical burden, caregiving burden (CRACB), psychological burden (PSYCH), nocturnal sleep disturbances (SDSTRB), and maladaptive coping strategies were collected at each assessment point. Results: Prior to CTh, 65% of dyads consisted of at least one poor sleeper, a rate further increasing to approximately 88% at CThC4. Multivariate hierarchical linear modelling revealed curvilinear trajectories for most dyads’ sleep/wake parameters that nevertheless reached significance (p<.05) for patients only. In both groups, sleep/wake impairment reached its peak at mid-treatment (CThC4); yet, patients consistently reported significantly more sleep problems than their carers. Partial convergence also emerged as suggested by positive correlations and no between-groups differences in daily disturbance, daytime napping duration, total sleep time, and overall sleep/wake impairment at pre-treatment. At CThC4, rates of change in sleep latency and daytime napping duration were also similar. In exploratory analyses, increased CRACB, poor SH, and SDSTRB consistently predicted poorer outcomes in the dyad members’ own sleep-wake patterns. Cross-partner effects most frequently emerged with regard to the dyads’ PSYCH, as well as for CRACB. Among the most interesting findings, increased patient PSYCH adversely affected caregiver perceived sleep quality and daytime napping. Reversely, increased CRACB was related to worse patient sleep quality, reduced sleep time, and difficulty to fall asleep. Some links might be suggested for own poor sleep hygiene and worse partner outcomes on daytime functioning, sleep efficiency, or wake after sleep onset. Conclusions and Future Implications: This is one of the first studies to show that a dyadic approach in the assessment of sleep/wake patterns in patients with breast cancer and their informal carers is a promising method to enhance exploration of potentially concurrent sleep/wake-impairment and associations with sleep-impairing factors that may co-vary in dyad members. Replication of the current findings in future dyadic sleep research is warranted. Meanwhile, clinicians will need to engage in concurrent systematic and ongoing sleep assessments that synthesise and contrast data to establish a care dyad’s level of sleep quality.