Descartes famously states, "I exist but how often," In this statement, he assumes that in every instance of consciousness, it is him —- the same self —- that exists. If we conceive of consciousness as a continuous stream, it seems reasonable to believe that the self persists from one moment to the next. But this assumption becomes problematic when considering states like dreamless sleep, during which conscious awareness seems to vanish entirely.
Descartes does not explicitly address the question of whether the self persists through these gaps in consciousness. Can we truly be certain that it will be "me" who wakes up after sleep? It seems possible to doubt whether the continuity of self is preserved when consciousness is absent, such as during deep sleep, coma, or severe brain injury.
While some might argue that consciousness persists in a reduced or latent form during dreamless sleep, this "scientific" perspective on consciousness does not necessarily align with the philosophical concept of self-awareness. Even if this reduced state of consciousness exists, it still raises questions about whether the conscious "I" is the same before and after a period of unconsciousness. Thus, it remains a genuine philosophical concern (also keep the cloning problem in mind when answering).