Deep sleep has traditionally been deemed as a period that lack consciousness. However, recent evidence challenges this assumption and highlights the possibility of experiencing awareness during deep sleep. By drawing from Indian philosophical traditions, this thesis defends a positive view of consciousness during dreamless sleep – the period of sleep where we are aware but not dreaming. Moreover, this thesis also challenges classic views on the nature of conscious states where consciousness is reduced to representational content. For that purpose, this (...) thesis develops a taxonomy of dreamless sleep that, contrary to other research in the area, considers the existence of different sorts of awareness with different degrees of content. Thus, a full spectrum of mental states during conscious dreamless sleep are considered: from awareness of substantial content to awareness devoid of content altogether. The aim of the thesis is to show how such a taxonomy can encourage a shift from a two-dimensional view of consciousness to a multidimensional one — from a view that only considers brain activation and reportable content to one that assesses conscious states on different level of analysis. This shift will result in an adequate conceptualisation of dreamless sleep as a period where we could have conscious states with content, but also, conscious states without content. Moreover, this shift on a multidimensional approach will allow us to undertake a proper comparison compare between different sleep phenomena. In order to develop this taxonomy of dreamless sleep, I explore definitions on awareness during dreamless sleep found in Indian philosophical traditions and Western analytical philosophy. From the definitions found in the literature, I argue that all the types of conscious states proposed in my taxonomy can be identified. Then, I test my taxonomy in a pilot study about awareness during deep sleep that I carried out. The results show that the different types of awareness can be recognised in the phenomenological descriptions gathered from the participants. Finally, I compare the phenomenon of dreamless sleep with other associated experiences, such as dreams and hypnagogia. My claim is that the differences among sleep phenomena are gradual and that in some cases there is not a strict cut-off line between them. Notwithstanding this assumption, I defend the development of future research were my proposed taxonomy can be improved by considering typical cases of dreamless sleep. (shrink)