You're conscious when you're dreaming ... I don't even know if you're sentient, not really. Usually sentience and consciousness can overlap in their definitions, but dreaming is a special case.
In all of the thread so far, I've been using the two terms interchangeably. That said, there is a LOT of the sleep period where you're not dreaming; dreaming is a subset of sleep, so if someone says "you lose consciousness when you sleep", their statement is true if you don't include the dreaming part. It's more charitable to just go with that axiom, even if you want to qualify-out dreaming. You should do that bit of distinction leg-work, just to speed on a discussion.
While sleeping, your brainstem can re-initiate consciousness and sentience; but you're not sentient during deep sleep.
To say that a sleeping person has sentience is a lot like saying that a standing person has a lap. If I told you a rock does not have a lap, you'd just agree. If I declared that a standing person has no lap ... you kind of squint for a second and wonder if you'll suggest that they have a lap ("right there! you can slap the front of your thighs! that's your lap!") or if they merely have the potential for a lap that a rock cannot ever have.