Variation in response to vaccination, particularly in vulnerable groups, provides a strong rationale for developing vaccine adjuvants. If there were consistent diurnal variation in immune response, this could inform a simple intervention for enhancing vaccine efficacy. Data from two studies are presented examining morning versus afternoon vaccine administration; in the first, hepatitis A vaccine was administered to young adults, and in the second, influenza vaccine to older community-based adults. Men, but not women, vaccinated in the morning mounted a better peak antibody response to both hepatitis A and the A/Panama influenza strain. These results indicate that it would be worthwhile testing this effect in a large randomized control trial with vaccination during time periods representing the extremes of hormonal and cytokine diurnal rhythms.