The investigation of sleep in disorders of consciousness (DoC) has shown promising diagnostic and prognostic results. However, the methods employed in this field of research are diverse. This leads to confusion in the way forward for both scientific and clinical purposes. We review the literature that has investigated sleep in DoC patients and specifically outline the methodologies used next to the presented results. We highlight what knowledge we currently have and where increased efforts are needed before further clinical implementation. Specifically, the review shows that successful methods may employ a two-stage approach to sleep scoring, where one is the application of loosened standard criteria and the other a more general factor describing closeness of the electroencephalography to a healthy pattern, including a score that describes the extent to which sleep scoring criteria can be applied. This should be performed as part of a multimodal approach that also includes investigations of eye-opening/closure and that of circadian (24-hour) rhythmicity. Taken together, this puts the most promising methodologies in the field together for a comprehensive investigation. Large-scale approaches, incorporating multiple modalities and looking at individual variation, are now needed to advance our understanding of sleep in DoC and its role in diagnosis, treatment, and recovery.
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