Postdoctoral researcher Mimi Shirasu-Hiza and her colleagues had previously found that fruit flies with abnormal circadian rhythms — the internal clocks than most animals, including humans, possess — had a particularly difficult time fighting off bacterial infections. In the current study, the team set off to determine whether the time of day that infection set in might affect normal flies' immune response.
...[T]he flies' immune systems reacted much better to two of the varieties of bacteria during the night...
Shirasu-Hiza raised two groups of flies in shoeboxes wired with tiny light bulbs so that she could control their day-night cycles. One group of flies was kept on a typical day-night cycle, but the second group's cycle was shifted by 12 hours so that it was exactly the opposite of the first group's. Using fly-sized syringes, Shirasu-Hiza injected her subjects with three different types of bacteria to see how their immune systems would react at different phases in the cycle.
What she found was that the flies' immune systems reacted much better to two of the varieties of bacteria during the night, resulting in better odds of survival for the flies. The activity of phagocytes, primitive immune cells that comb the body for bacterial invaders and then devour them, was much higher at night than during the day. Additionally, in mutant flies who had no circadian cycle, the activity of phagocytes was much reduced compared to normal flies.
Shirasu-Hiza says that her results "suggest that immunity is stronger at night, consistent with the hypothesis that circadian proteins upregulate restorative functions such as specific immune responses during sleep, when animals are not engaged in metabolically costly activities." In other words, at nighttime, when the body is not expending energy on more lively pursuits (finding food, mates, etc.), it can spent that time boosting its immune reaction to fight off the parasites that are constantly trying to invade it. Whether the results of the study are pertinent to human remains to be seen, but the authors say it is a possibility.