A Narrative Review on How Timing Matters: Circadian and Sleep Influences on Influenza Vaccine Induced Immunity.

We aimed to synthesize and critically evaluate human studies on the impact of circadian and sleep factors on influenza vaccine-induced immune responses. A comprehensive literature review was conducted, and of the 1260 studies identified, 13 met the inclusion criteria for evaluating vaccination timing, circadian misalignment, and sleep parameters in relation to influenza vaccine-induced immune responses in human populations. Most studies assessed humoral immune responses, primarily antibody titers. Morning vaccination (typically between 9:00 and 11:00 AM) was associated with higher antibody titers compared to afternoon vaccination, particularly for the A/H1N1 strain in adults aged ≥ 65 years. Short sleep duration-especially in the two nights preceding vaccination-was associated with reduced antibody levels, while acute sleep deprivation the night after vaccination transiently reduced antibody levels in males. Sleep fragmentation and excessive daytime sleepiness were linked to increased vulnerability to breakthrough infections. Evidence on circadian misalignment from shift work was mixed. Clinical outcomes were reported in one large trial, where morning vaccination correlated with fewer respiratory hospitalizations. Current evidence supports a potential role for circadian timing and sleep duration in enhancing vaccine-induced antibody responses, particularly in older adults and individuals with sleep or circadian disruption. However, inconsistencies, modest effect sizes, and methodological limitations preclude broad recommendations. Future studies should incorporate direct measures of circadian phase, stratify by chronotype and population (e.g., shift workers), and evaluate both immunologic and clinical outcomes to inform targeted chrono-immunization strategies.