This is probably a stupid and question and it's probably going to get taken down but whatever. While I was reading through some of the questions and I stumbled upon a question about biological engineering, and that got me thinking, "How would people create a functioning society inside of a biological being?" Let's just assume that this being has the biology and the of a human and the people have modern technology. Also, the size and the way this society behaves is up to interpretation. I know this is a really ambiguous question and I apologize in advance.
A society needs food, water and shelter for basic survival.
This civilization is parasitic. They'd have to consume parts of the host's body as food. Those humans living near intestinal tracts or the mouth might obtain plant matter on its way towards digestion, as the rest of the mini-humans would have only meat food sources. Others might live in the fat reserves such as on the belly.
Some process could be derived to extract water from blood, or other cells.
Shelter gets interesting, as the only building materials still come from inside the host. Slivers of bone are shaved away to erect housing structures, or else small holes are bored in between
muscles or other organs.
The mini-humans need oxygen, which is supplied via the super -human's blood stream.
Individual residences might be located next to arteries to access passing resources.
Maybe they have machines to automatically 'farm' blood, breaking oxygen off hemoglobin and pumping it into the houses. You mentioned the human being asleep, but perhaps, some mini-humans live along nerve cells to fire them and control the host body, while others observe and relay surrounding circumstances to them.
Criminals might be ejected via the lower intestines.
Travel is most likely through the blood stream. A small craft could float throughout the body, stopping at certain connection points. Returning to the starting point would require going all the way through the heart again.
The biggest issue are the scaling effects – do the small humans have small atoms and molecules or do they just have fewer of the big atoms and molecules? But assuming all scaling effects can be ignored there are a range of different possibilities depending on size and location. The size would need to be small enough that they could construct a reasonable sized habitat, but big enough to allow the suspension of disbelief of scaling effects to be plausible.
It is fortunate that the little people have modern technology. Assuming when they arrive that they are well prepared, I think they would need a habitat to live in probably best to have a strong pressure vessel to withstand jolts, knocks and movements of the big human. They would also need a safe place to lodge their habitat. The blood stream would not be good as they would be continuously tumbled and might get stuck somewhere causing a blockage and an aneurism in the big person. I would suggest somewhere deep in fatty tissue would be a good place near a small vein.
All of the components necessary for life would be available outside of the habitat, but to avoid creating severe irritation to the big human they would have to be careful not to disturb their environment too much and they would need a lot of sophisticated equipment to meet their needs.
They will need power to process materials ideally a very small nuclear unit or failing that perhaps some sort of turbine in the blood flow.
Oxygen and carbon dioxide
Oxygen could be extracted from water by electrolysis and carbon dioxide could be scrubbed from the atmosphere and chemically combined with the hydrogen from the electrolysis to produce some organic substance that was not too toxic and could be released into the blood stream as waste. Alternatively the might be able to bring in some red blood cells to create their own mini lung.
If the little people had strong stomachs they could harvest a few fat cells, blood cells and muscle cells and use these for food. Preparation and cooking would not be easy and disposal of waste would be particularly difficult if they were intent on avoiding creation of irritation.
One solution would be to fill up some sort of cellular sack with waste and jettison it into the blood stream ensuring that the package was small enough not to create any blockage. In this way the blood stream would move the waste away from the habitat so avoiding irritation at that site and the body would deal with the waste like any other dead cellular matter.
The little people might be able to travel by using scuba gear when outside the habitat but they would have to be very careful not to be carried way in the blood stream.
As Vincent has pointed out a civilisation requires several basic necessities to survive. One option is that this civilsation is parasitic 'stealing' from the host body the food, water, oxygen and living space. Another option is that they are symbionts required for the host body like we require our bacterial cultures in our intestines and on our skin to survive.
The inhabitant would have to exist on a small enoguh scale to move through blood vessels without filling them up completely. Consider a large host creature which is inhabited by a 'nanite' society which lives of his ressources, but at the same time repairs injuries, fights off bacteria and viruses. Water is an abundant ressource in such an environment, food could be filtered out of the blood stream directly, extracted from stomach or intestines or from the cells of these organs responsible for extracting nutrition. If you live within the blood stream and extract oxygen from it directly you might not have need for specific shelter or the host having adapted its body to this symbiosis over time has developed a special gland to provide shelter for his society of miniscule helpers.
This scenario reminded me of the educational mini series "Once Upon a Time... Life" where cell in our body with specific functions like immune cells are represented as people fighting bacteria.