New research suggests that flu shots may be more effective when people get them in the morning as opposed to the afternoon.

British researchers assessed 276 people 65 and older who received vaccinations against three different flu strains between 2011 and 2013. The patients received the vaccines either between 9 am and 11 am, or 3 pm and 5 pm.

People in the morning group had a much larger increase in antibodies against two of the flu strains one month after vaccination, the researchers found. However, with the third flu strain, there was no significant difference between the morning and afternoon groups.

“We know that there are fluctuations in immune responses throughout the day and wanted to examine whether this would extend to the antibody response to vaccination,” said lead investigator Anna Phillips. She’s with the University of Birmingham’s School of Sport, Exercise and Rehabilitation Sciences.

“Being able to see that morning vaccinations yield a more efficient response will not only help in strategies for flu vaccination, but might provide clues to improve vaccination strategies more generally,” Phillips said in a university news release.

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