Here’s a relevant fragment form the Maw (spoilers).
“[..] He was subjected to the Dementor's Embrace.”
“You mean he lost his soul?” Harry asked, the thought beginning to chill him.
“No,” said Dumbledore. “A Dementor's Embrace is not like a Dementor's Kiss. ███'s soul was not devoured by the Dementor, but was actually imprisoned. His soul is trapped, and cannot escape.” He shook his head, stroked his white beard, and then sighed. “███ is neither alive nor dead. When the Dementor that you had summoned Embraced him, it performed a sort of transformation that I cannot even begin to understand. In a few decades, ███ will no longer be ███. He will eventually become a Dementor.”
All of Harry's unanswered questions unraveled themselves. The origin of the Dementor. “What? I've never heard of a Dementor doing that before.”
“That's because there are very few people who are in such a condition as to allow such a transformation to happen,” said Dumbledore. “In order for a Dementor to perform an Embrace, the victim would have to be in such a weak state of mind that the Dementor could easily ensnare it to its own purposes. ███'s unfortunate life and history made it easy for him to succumb to the Dementor's powers.”
“So you're saying that all of the Dementors in the world were once human?”
“Yes,” answered Dumbledore. “Every last one. How the first one was created, I do not know. But all of the successive Dementors were once incredibly depressed and pained individuals, who were twisted and corrupted into the Dementors that you know now.”
Somehow, everything about this seemed wrong to Harry. He could not put his objections into words, however. “So we're going to—to let him become a Dementor?”
“The Dementor's Embrace leaves the victim incapacitated, Harry,” continued Dumbledore. His voice, so soft and gentle, sounded unfit for the topic of conversation. “███'s waking hours for the next decades will be very few. Until he becomes a full Dementor, he will remain unconscious and dead to the world. Excepting very few occasions.”
“What do you mean about 'very few occasions'? You mean he can wake up?”
“During his unconscious state, ███ will be reliving his worst nightmares,” continued Dumbledore. “These nightmares, in fact, are caused by the Embrace. The Dementor literally eats all of the victim's joyful memories, leaving, them with nothing but the worst horrors of their lives. As individuals with terrible pasts tend to have fewer positive memories, you can see why the Dementor's Embrace is most common among such people. To answer your question, while ███ has these nightmares, there will be—in fact, has been—periods where he will become roused, and will mindlessly wander. Sometimes these occurrences are peaceful, sometimes not. Do not ask me for the specifics. The subject is currently being studied by the best minds in the Department of Mysteries.”