Hematological malignancies remain one of the leading causes of morbidity and mortality despite advances in targeted therapies, immunotherapy, and stem cell transplantation. Emerging evidence indicates that treatment efficacy and toxicity depend not only on the choice of therapy but also on its timing relative to the patient's internal circadian rhythm. The circadian clock orchestrates fundamental processes in hematopoiesis and immunity, such as stem-cell proliferation, leukocyte trafficking, DNA repair, and drug metabolism, while its disruption promotes malignant transformation, therapeutic resistance, and systemic toxicity. This narrative review synthesizes current understanding of circadian regulation in hematopoietic and immune systems, the mechanistic and preclinical foundations of chronotherapy in blood cancers, and the limited but growing body of clinical evidence linking treatment timing with outcome in leukemia, lymphoma, and transplantation. The review also examines practical challenges, including inter-individual variability, disease-induced circadian disruption, and hospital workflow constraints, while highlighting emerging technologies, such as transcriptomic clocks, wearable biosensors, and AI-driven scheduling algorithms, that are poised to enable personalized, time-aware therapy. By integrating temporal precision into existing therapeutic frameworks, chronotherapy may represent a promising investigational dimension of precision medicine in hematological oncology. However, its clinical value remains to be defined through prospective studies that incorporate validated circadian biomarkers, predefined timing windows, and clinically meaningful efficacy and toxicity endpoints.
