Circadian rhythms are key regulators of immune functions. These endogenous oscillations help to maintain immune homeostasis, regulate responses to pathogens, and shape vaccine efficacy. Recent studies further indicate that they are of clinical relevance for cancer immunotherapies. While circadian immune rhythms are thus recognized to be important in adults, it is unknown at what developmental stage these rhythms begin to manifest. In this opinion article we review the development of circadian rhythms in the immune system in both rodents and humans, with a focus on their interactions during the perinatal period. Understanding their emergence in early life may help guide time-based clinical interventions for infants.