What can we learn about brain functions from sleep EEG? Insights from sleep of DOC patients

Disorder of Consciousness (DOC) patients are often reported to have alterations in sleep architecture and sleep-specific graphoelements. The reappearance of non-REM oscillatory patterns such as sleep spindles has been associated with diagnosis and presumably prognosis. The study of sleep is of particular interest in DOC research as it might allow identifying lesioned neuronal tissue linked to specific sleep graphoelements. The presence of REM, for example, may reflect residual functioning of brainstem nuclei including pons and adjacent portions of the midbrain. On the other hand, the absence of circadian functioning, or sleep-wake cycles, has been associated with brainstem dysfunction and might be informative for hypothalamic and SCN alterations. Interestingly, the systematic work on sleep and circadian functioning in the various DOC states is still scarce. Most strikingly, there are no accepted criteria which allow to reliably classify sleep stages in these patients. In addition, there is very little knowledge whether sleep in DOC is still under normal circadian and homeostatic control. Without a doubt, studying sleep in DOC is a challenging endeavor, and long-term polysomnography (PSG) of DOC patients is difficult for many reasons, among which are ubiquitous artifacts present in the data recordings or PSG alterations which often come with pharmacological treatment. Altogether, the characterization of sleep and its relationships with arousal, homeostasis, and circadian rhythmicity is, in our view, essential in order to better understand the various DOC states and might offer complementary diagnostic and prognostic information or even guide the direction of future treatment attempts.