Flu vaccines are more effective when given in the mornings, research suggests.
A new study of 276 people found a better bodily response to vaccines if they were given before 11am, suggesting a higher level of protection.
The study, published in the journal Vaccine, saw over-65s vaccinated against three strains of flu, either during morning GP surgeries (9-11am) or afternoon surgeries (3-5pm).
For two of the three strains, there was a significantly bigger increase in antibody concentration one month after vaccination for those receiving the jab in the morning, compared with those given the jab during the afternoon.
For the third strain of flu, there was no significant difference between morning and afternoon sessions.
The authors concluded: "The present study was the first large-scale randomised trial of different times of vaccination and provides evidence that morning vaccination enhances the antibody response to the influenza vaccine."
Dr Anna Phillips, from the School of Sport, Exercise and Rehabilitation Sciences at the University of Birmingham, who led the study, added: "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.
"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."
Overall, 24 GP practices across the West Midlands took part in the study, which ran between 2011 and 2013.
Figures released earlier this month showed that deaths in England and Wales hit a 12-year high last year, with flu thought to be to blame for taking the lives of a significant number of people aged 75 and over.
The substantial rise in deaths has been partly blamed on the lack of effectiveness of that season's winter flu vaccine.
The vast majority of the extra deaths in 2015 were registered in the first three months of the year, coinciding with a peak in flu.
There were 24,065 more deaths in the first three months of 2015 compared with the same period in 2014, with 11,865 of these extra deaths registered in January alone, when flu was circulating at its highest levels.
Professor Janet Lord, a co-investigator on the new study, from the Institute of Inflammation and Ageing at the University of Birmingham, said: "A significant amount of resource is used to try and prevent flu infection each year, particularly in older adults, but less than half make enough antibody to be fully protected.
"Our results suggest that by shifting the time of those vaccinations to the morning we can improve their efficiency with no extra cost to the health service."
Dr. Richard Pebody, head of flu surveillance for Public Health England, said: "This is an interesting study and indicates more research is needed.
"Flu vaccine is the best protection we have against an unpredictable virus which can cause severe illness and deaths each year among at-risk groups, including older people, pregnant women and those with a health condition, even one that is well managed."
Experts cautiously welcomed the study but said more research was needed.
Adam Finn, professor of paediatrics at the University of Bristol, said it would be "important to assess whether these early results translate into better protection over time."
He added: "The authors are likely to be planning these further studies which would need to be done before making any recommendations to change current practice on timing of vaccinations."
Jonathan Ball, professor of molecular virology at the University of Nottingham, said: "We know that a variety of factors, such as diet, sleep and exercise seem to impact on your immunity.
"But the fact that the amount of antibody produced following influenza vaccination differed according to whether or not the people included in the study were immunised in the morning or in the afternoon was intriguing.
"But we have to remember - differences in antibody yield that are statistically significant might not be biologically significant, and that's the key issue.
"We would need to see whether or not people vaccinated in the afternoon were more likely to become infected by the virus before we could say that timing of immunisation impacts on success."
Dr Rachel Edgar, from the University of Cambridge, said: "This robust study shows that our antibody responses to seasonal flu vaccines depend on the time of day at which they are given, and suggests vaccines may be more effective when administered in the morning, rather than afternoon.
Further investigation was needed to determine if morning vaccination strategies reduce the incidence of disease, and how vaccine efficacy is governed by our circadian (24hr) rhythms, Dr Edgar said.