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It is well known that some clinical signs and symptoms of rheumatoid arthritis (RA) vary within a day and between days; the morning stiffness that is observed in patients who have RA has become one of the diagnostic criteria of the disease. The circadian changes in the metabolism or nocturnal secretion of endogenous corticosteroids is certainly responsible, in part, for the time-dependent changes that are observed in the inflammatory response and related clinical symptoms. More recently, melatonin (mLT), another circadian nocturnal hormone that is the secretory product of the pineal gland, has been implicated in time-dependent inflammatory reactions, with effects that are opposite of those of corticosteroids. Therefore, altered functioning of the hypothalamic-pituitary-adrenocortical axis (reduced corticosteroid production) and of the pineal gland (increased mLT production) found in RA patients seem to be important factors in the perpetuation and clinical circadian symptoms of the disease. Consistently, human proinflammatory Th1-type cytokine production (related to mLT stimulation) exhibits a diurnal rhythmicity, with peak levels during the night and early morning, at a time when plasma cortisol (inducing Th2-type cytokine production) is lowest and mLT is highest. Reduced daily light exposure as observed in northern Europe (Estonia), at least during the winter, might explain the higher and more prolonged mLT concentrations as well as some epidemiological features that are observed in northern European patients with RA versus southern European patients.