Adoptive cell therapies (ACTs), such as chimeric antigen receptor (CAR)-T cell therapy, have revolutionized cancer treatment, especially for hematological cancers. However, patient responses vary considerably. Emerging research reveals a striking influence of time of day (ToD) on ACT efficacy. Administering ACT during the early behavioral active phase enhances tumor control and reduces toxicity in preclinical models, an effect linked to the circadian clock. Latest clinical data also point to ToD effects in the cancer setting. In this opinion article we explore current insights and discuss the emerging underlying mechanisms. We propose that integrating ToD into clinical practice could represent a powerful yet easily implementable therapeutic regimen to improve efficacy and safety of ACT.