Interactions between sleep, other body rhythms, immune responses, and exercise.

Biological rhythms can influence both exercise tolerance and immune function. Most studies have focussed upon circadian rhythms, but some circaseptan, circatrigintan, and circannual cycles have also been described. Rhythms may have an endogenous or an exogenous basis. Endogenous rhythms originate in the cells of the suprachiasmatic nucleus. Impulses from this region pass to the pineal gland, where they regulate the production of melatonin, a compound that modulates the sleep/wakefulness cycle. Endocrine, physiological, and psychological parameters all show evidence of periodicity. Most components of the immune system, both cellular and humoral, also show large rhythmic changes. It is not yet clear how far such periodicity is secondary to changes in other body systems. Nevertheless, the magnitude of variation is such that it is vital to obtain experimental and control data at identical time points in both acute and chronic studies of exercise and immune function. Disturbance of the sleep/wakefulness cycle can constitute a form of stress, with adverse consequence for immune function.