Is there consciouness when a person is sleeping and not dreaming?
According to Giulio Tononi,"consciousness is what goes away when you fall into a dreamless sleep,” he explained that  "once a person goes into dreamless sleep, there is no experience, no perception of surroundings or the environment; there are no memories of the past or the future or even the present. It is thus at this point that everything disappears, which corresponds to consciousness “going away.”" [1]
You can look up his integrated information theory of consciousness [2]


Tononi G, Edelman GM: Consciousness and complexity. Science. 1998, 282 (5395): 1846-1851. 10.1126/science.282.5395.1846.
Tononi, G. (2011). The integrated information theory of consciousness: an updated account. Archives italiennes de biologie, 150(2/3), 56-90.