Flu jabs 'more effective in morning'
Morning flu jabs provoke a stronger immune response than those given in the afternoon, a study shows.
The trial at 24 doctors' practices found people vaccinated before lunch produced the most defensive antibodies.
The University of Birmingham team suggested immunising people in tune with the body's natural rhythm could be a cheap way to save lives.
Experts said the study may mark the dawn of making use of "the body clock in the clinic".
Our internal clock alters our alertness, mood, physical strength and even the risk of a heart attack in a daily rhythm.
And our immune system also waxes and wanes through the day.
The trial looked at 276 healthy people, aged over 65, getting the flu jab before the 2011, 2012 and 2013 flu seasons.
They were vaccinated either in a morning session (09:00 to 11:00) or an afternoon appointment (13:00 to 17:00).
One month later, patients vaccinated in the morning had produced significantly more antibodies against two of the three flu strains in the jab.
Similar antibody levels were produced for the third strain, the results in the journal Vaccine showed.
Dr Anna Phillips, one of the researchers from the University of Birmingham, said the results were meaningful and doctors should "definitely" think about performing flu jabs in the morning.
She told the BBC News website: "A lot of surgeries just try and fit in vaccination anyway so it's not going to risk any patient, it's not going to cost anything and even if we're wrong you've nothing to lose by doing this.
"I think it's fantastic, the idea of an intervention this easy to do and free is unheard of in terms of trying to change NHS practice."
It is not clear exactly what the critical difference between the morning and afternoon immune system is.
Levels of immune messengers called cytokines, the stress hormone cortisol and sex hormones - all of which affect the immune system - fluctuate in a daily rhythm.
And individual white blood cells also have their own internal clocks that alter their activity too.
Andrew Loudon and David Ray, a pair of body clock professors at the University of Manchester, told the BBC News website: "This may be the dawn of the body clock in the clinic.
"This is a most interesting study, and is among the first to show how the body clock can be used to make healthcare interventions more effective.
"There have been major advances in understanding how the body clock can regulate immunity in laboratory animals, but very little of that exciting science has led to changes in healthcare.
"This study shows that a simple intervention, giving the same vaccine at a different time of day, can result in a major gain in effectiveness."
However, other vaccines stimulate the immune system in different ways so it is too simple to conclude that all immunisation should take place before lunch.
There have been some suggestions that hepatitis B vaccination may be more effective in the afternoon.
But the concept of timing medicine to the body clock - the field of chronotherapy - is powerful and is also showing promise in treating cancer and rheumatoid arthritis.
Follow James on Twitter.