Sleep is a complex and essential activity for the preservation of the brain and for the general health of an individual. The nature of sleep is therefore of major importance for brain injured patients with disorders of consciousness. However the standard definition of sleep is not relevant for this clinical population and it is thus necessary to adapt the standard method of sleep analysis. In this book we review the specialized literature and present our own original study of sleep in the unresponsive wakefulness syndrome and minimally conscious state. We describe the wide variety of sleep patterns possible in these patients who often no longer retain their sleep-wake cycle. In particular we shed light on the fact that standard sleep stages can differentiate the minimally conscious state from the unresponsive wakefulness syndrome and that they predict possible behavioral improvements within a period of 6 months. This new and promising field of research should be especially useful to neurologists and researchers who wish to improve the clinical evaluation and care of their patients as well as our understanding of the neural correlations of vigilance and consciousness.