Dreaming begins when a person enters R.E.M (Rapid Eye Movement) sleep, which tends to be around 90 minutes after they fall asleep. While there is no generally accepted explanation for why we dream, it is evident that without entering REM sleep we quickly lose the ability to function. People who have gone several days without entering rem sleep often experience hallucinations, shortened attention spans, an inability so perform complex mental functions, and occasionally physiological reactions. Dreams can seem to last hours or days, but in actuality they rarely last for longer than ten minutes. This, more than anything, is a testament to the human brains untapped potential, if our subconscious is able to play out fantasies of such detail that much faster than we could ever experience them in real life, than the brain is capable of compiling and compressing more data in its free time than all the supercomputers in the world. This also means that if our favorite dream demon can send these images to us, then he is capable of processing as much as our minds can handle on a consinouns and deliberate level. There are many theories as to the purpose of dreams, including the ideas that they effect our long term memory, how we perceive individual memories, and how we remove unused emotions or sensations, these theories are not all bound together, but neither are they mutually exclusive, meaning if one had power over dreaming, he could possibly effect all these other mental functions as well.
Dreaming also has psychological implications. Dreams are often seen as an opportunity to play out different scenarios in our head, with improved understanding of the results but no risk to ourselves. Many people have claimed that dreaming has provided them with answers to problems that they couldn't solve while awake, showing another example of how much potential we have locked away. Our favorite pervert, Freud, believed that dreams were a chance for the Id to live out its fantasies without the repercussions that would normally keep the ego or superego from stopping it. He also believed that dreams were a way to conquer our emotions in stressful or confusing situations.
Dream interpretation is a heavily explored and long existing field of study, bordering on the realm of accepted science. Many notable psychologists and ancient civilizations have contributed to the study of understanding dreams. Despite all of these, or perhaps because of them, we are often left with many different interpretations for each dream. One person could claim that a single dream represents several different issues manifesting themselves in a single scene, and someone else could say that the same dream focuses solely on a single issue, unrelated to the other person’s interpretations. The fact is, to fully understand most dreams you need a lot of context, education, and practice, and I have very little of any in this field, so I'm not even going to try to fully explore this topic.
EDIT: A new note, one that is primairly speculation at this point, but I have found enough information that I feel comfortable talking about it here with the proper disclaimer. There is evidence that the Slender Man is not the only creature capable of entering people's dreams. At this point what evidence I have found leans towards the theory that whoever or whatever intiates the dream has the most power over the reciver. This means that, if I am correct, Slender Man and creatures of a similar nature do not actually enter your dreams, rather they "manufacture" dreams, which they then place in your mind for you to view and interact with. again this is just a theory, and it does not diminish the effects a dream can have on your mind, it is just an explination of how these dreams can occur. I think that He can still "enter" dreams, but he does not have as much control in this situtaion as it is still "your dream." Take from this what you will, it is hardly a breakthrough, but I always love stumbling across new information. If you have any new ideas or evidence let me know.
Well that's all I have on the topic of dreaming for now, though new discoveries do happen from time to time, and every now and then my research will reveal some new light on a topic I have already explored, so it is entirely possible I will return and improve this information from time to time. I know for a fact my roommate just committed to writing a paper on this subject, so maybe I'll steal some of his work when he is finished. Also, please do share some of your own dreams below, I'm interested in seeing what all of your experiences are.
See you around