Does consciousness disappear in dreamless sleep?

Consciousness is often said to disappear in deep, dreamless sleep. We argue
that this assumption is oversimplified. Unless dreamless sleep is defined as
unconscious from the outset there are good empirical and theoretical reasons
for saying that a range of different types of sleep experience, some of which are
distinct from dreaming, can occur in all stages of sleep. We introduce a novel
taxonomy for describing different kinds of dreamless sleep experiences and
suggest research methods for their investigation. Future studies should focus
on three areas: memory consolidation, sleep disorders, and sleep state (mis)
perception. Our proposal suggests new directions for sleep and dream science,
as well as for the neuroscience of consciousness, and can also inform the
diagnosis and treatment of sleep disorders.