This study by Drs. Benjamin Natelson and Fumharu Togo looked for a relationship between heart rate variability (HRV) and subjective sleep quality. The 52 female patients with CFS showed evidence for sleep disruption in the form of significantly reduced total sleep time, reduced sleep efficiency, and changes in whole night HRV when compared to matched healthy controls. “Our study showed that cardiovascular regulation of CFS patients differed from that of healthy controls even when we eliminated effects of sleep macro-architecture. This could be a reason why CFS patients report their sleep to be unrefreshing.” (Autonomic Neuroscience, Mar. 18, 2013)