In addition, circulating inflammatory mediators such as interleukin-6 (IL-6) may also be involved, as studies in healthy controls have shown that higher concentrations are associated with fatigue, insomnia and sleep deprivation (Mastorakos et al., 1993; Vgontzas and Chrousos, 2002; Vgontzas et al., 1999, 2002). Here, IL6 is linked to insomnia.