Figure 1

Rheumatoid arthritis shows diurnal variation in disease symptoms and markers. Joint stiffness and pain are more pronounced in the early morning [1], and this correlates with the early morning rise in plasma IL-6 levels [3]. The circadian hormone melatonin (which is considered to exacerbate the inflammatory response) is released only during the night, and circulating levels peak in the mid-night. The anti-inflammatory glucocorticoid - cortisol - is also under circadian control, peaking in the early morning.