The central nervous biological CLOCK system (highly conserved and sophisticated molecular 'clock'), under the influence of light/dark alterations, 'creates' the internal circadian rhythms. The organisms 'feel' these rhythmic external changes to synchronise their physical activities, including energy metabolism, sleep, and immune function. A number of immunological functions are dependent on influence of sleep on circadian rhythms, including the type and magnitude of immune responses following antigenic challenge. Loss of sleep, in turn, prevents these immunosupportive actions and alters the production of glucocorticoids during the night. Major life events lead to an intense release of stress response system mediators (mainly norepinephrine and cortisol), whereas in minor life events, only short-lived surges of these neurotransmitters and hormones are expected. The immune system reactivity follows circadian rhythms imposed by the CLOCK and sleep synchronisation, and is particularly altered in presence of chronic stress. As a consequence of the altered CNS-endocrine control, low-dose long-term glucocorticoid therapy in chronic rheumatic diseases is today considered as a 'hormonal replacement therapy' to supplement the peripheral insufficiency of endogenous glucocorticoids in modulating the immune/inflammatory reaction.