POMC and myalgic encephalomeyelitis/chronic fatigue syndrome: Putative common causes for abnormal sleep and increased CFS/ME susceptibility revolve around the hypothalamic–pituitary–adrenal (HPA) axis function [28], [29], manifesting as differences in cortisol [30], [31], adrenocorticotropic hormone [32] and melatonin [33], and around the autonomic nervous system function, manifesting as differences in heart rate variability [34], [35], [36], [37].