Experiments with the laboratory model organism, Drosophila melanogaster, reveal that the specific immune response known as phagocytosis oscillates with the body's circadian rhythm -- 24-hour cycle that sets the rest/activity cycle, Mimi Shirasu-Hiza of Stanford University colleague David Schneider said.

"These results suggest that immunity is stronger at night, consistent with the hypothesis that circadian proteins regulate restorative functions such as specific immune responses during sleep, when animals are not engaged in metabolically costly activities," Shirasu-Hiza said in a statement.

In previous experiments, the researchers noted that fruit flies sick with bacterial infection lost their circadian rhythm and that flies lacking circadian rhythm were highly susceptible to infection.

The flies were infected with two different bacterial pathogens, Listeria monocytogenes and Streptococcus pneumoniae. The study found the fruit flies infected at night had a better chance of surviving than did the flies infected during the day.

The American Society for Cell Biology 48th annual meeting in San Francisco.