Whole-body malaise and fatigue associated with illness are linked to innate immune system activation releasing interleukins that interact with the dorsal raphe nucleus, increasing serotonin and inducing sleepiness.

“that whole body malaise that extra fatigue that's not easily explained by other factors in your life have to do with the fact that when your innate immune system is activated meaning it's already combating a cold or flu interlukin 1 and interlukin 6 have a way of interacting with a particular brain area called the dorsal rafay nucleus which is choa block full of neurons that release serotonin and serotonin from the dorsal rafay nucleus acts on specific regions of your hypothalamus areas like the preoptic region for those of you that want to know and other areas of the hypothalamus that generate a state of sleepiness”