First remember that there are distinctive differences between acute pain and chronic pain, and that a history of chronic pain can complicate the management of acute pain.
For a person experiencing acute pain, I would not expect them to be able to sleep with a pain level of 8, 9, or 10. By definition, that is extreme pain that prevents normal functioning. A 10/10 acute pain would be like having your arm ripped off or being mauled by a bear or being submerged in boiling oil. It's the worst possible pain there can be. It's so severe that you actually pass out. I have seen very few people in 10/10 pain, but the ones I have seen, I will never forget. These are people with blood pressures of 285/150 while grimacing and bucking the tube on astronomically high doses of Propofol, needing liberal and frequent doses of Fentanyl. I have seen 10/10 acute pain, and when someone (without a history of chronic pain) is sitting there texting on their cell phone and smiling, telling me their pain is 12/10, forgive me if I have a hard time believing that.
With chronic pain, the body adapts to being in pain over time; there are actual changes to the brain and nervous system that occur, where the person can be at a very high level of pain, but their vitals are WNL and their behavior does not "look painful". What also can happen is that the person A) Develops a tolerance to opiates, requiring larger and larger doses, over time, to achieve the same effect, and B) Develops an extreme sensitivity to pain, where things that might not seem very painful in ordinary circumstances, can be excruciating to the person.
When the person with chronic pain experiences acute pain, the acute pain might be more intense for them that it would be for the person without a history of chronic pain. Also, they may have developed a high tolerance for opiates, so much so, that you could give them enough Dilaudid to kill an elephant, and they're still going to be hurting. They might not "look" painful, because they're always painful. Their VS might be WNL, because they have adapted physiologically.
So, as a general statement, the idea that a sleeping person is not experiencing pain is false. While it may be true for the person experiencing acute pain, the person who suffers from chronic pain has learned to sleep even when experiencing a high level of pain.
I think you are right to look at the whole picture and ask yourself some questions. Look at the patient's history, what they normally take for pain, and talk to them.
Whenever I'm confronted with someone who's experiencing acute on chronic pain and has been on opiates forever and a day, I just level with them. I tell them that I will do my best to get them more comfortable, but that their safety is my primary concern. I tell them "I don't want you to be in pain, but I don't want to kill you, either.". I will give what medications I can safely give, because I don't want them to hurt, but most likely, we're not going to get them pain free; and generally, chronic pain sufferers don't expect to be pain free, ever. I find out what level they are currently at, and ask them what level they would like to be at. If they live at a 6 but they're currently at an 8, I'll try to get them to a 6, but they might have to settle for a 7. I show them that I am involved and invested in their pain control, but that their safety is just as important to me, and that we need to be realistic in our thinking about what we can achieve. Most people who live with chronic pain are very receptive to this, and appreciate my concern for both their pain control AND their safety.
I can have the same kind of conversation with the person experiencing acute pain, but the concept that they will probably not be at a "0" any time soon is not usually a concept they've thought of. Often, the person experiencing acute pain has a belief that being pain free is a reasonable goal, when it is often not. So that's where the conversation usually starts, with clearing up that misconception. Then we go from there and work together to set a goal that seems achievable, and at which they can function and participate in their recovery.