Chronobiology of the Immune System

Activation, proliferation and circulation of immune cells vary predictably according to circadian (approximately 24-hour) rhythms. These biological rhythms are endogenous and genetically based. Rhythms in nocturnally active experimental animals such as mice or rats are 12 hours out of phase from those in diurnally active humans. In animal studies, the toxic and immunomodulatory effects of agents such as interferon, interleukin-2 and cyclosporin can vary by 50% or more according to time of administration, presumably as a result of circadian rhythms.