IL6 and rheumatoid arthritis: There is diurnal variation in symptoms of RA, with exacerbations of joint pain and stiffness in the morning, aligned to rhythmic expression of pro-inflammatory serum cytokines such as interleukin 6 (IL-6), highlighting the role of circadian rhythms in RA inflammation (Gibbs and Ray, 2013; Perry et al., 2009; Arvidson et al., 1994).