Will the time affect medicine?The time of the day could be an important factor in the risk of getting an infection, according to researchers in the US.
They showed how a protein in the immune system was affected by changes in the chemistry of the body through the day.
The findings, published in the journal Immunity, showed the time of an infection changed its severity.
An expert said drugs were likely to take advantage of the body clock in the near future.
Plants, animals and even bacteria go through a daily 24-hour routine, known as a circadian rhythm. Jet lag is what happens when the body gets out of sync with its surroundings after crossing time zones.
It has been known that there are variations in the immune system throughout the day. Researchers are now drilling down into the details.
The immune system needs to detect an infection before it can begin to fight it off. Researchers at Yale University School of Medicine were investigating one of the proteins involved in the detection process - Toll-like receptor nine (TLR9), which can spot DNA from bacteria and viruses.
In experiments on mice, the scientists showed that the amount of TLR9 produced and the way it functioned was controlled by the body clock and varied through the day.
Immunising mice at the peak of TLR9 activity improved the immune response, the researchers said.
They said humans with sepsis, blood poisoning, were known to be at a greater risk of death between 02:00 and 06:00.
Time linkWhen testing mice, the severity of sepsis depended on the time of day infection started and coincided with changes in TLR9 activity.
Prof Erol Fikrig, who conducted the study at Yale University, said they had found a "direct molecular link between circadian rhythms and the immune system", which could have "important implications for the prevention and treatment of disease".
He added: "It does appear that disruptions of the circadian clock influence our susceptibility to pathogens."
Dr Akhilesh Reddy, who is researching circadian rhythms at the University of Cambridge, said it was "known long ago" that timing had an impact on the immune system, but this was "one of the first forays" into the reasons why.
The implications for healthcare could mean that drugs need to be given at certain times of day in order to make them more effective, or drugs could be made which actually target the body clock to put the immune system into its most active phase.
Dr Reddy said drug companies were "all switching onto this" and were "now screening drugs at different times of the day".
He could see the body clock impacting medicine "within 10 years".
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