I learned about sleep cycles from chapters in books on lucid dreaming, for example books by Stephen LaBerge or now I am reading a book by Daniel Love and he also goes over the basics of sleep cycles.

Here is a web link to something about sleep cycles but there is tons of other info out there, some scientifically and some less so. Stages of Sleep: REM and Non-REM Sleep Cycles

Personally I am not as much into the "beyond dreaming" stuff like what if it is another realm, but I know other members of this forum are, check out the Beyond Dreaming section if this interests you. Although I do admit being sometimes intrigued or amused by beyond dreaming subjects, and I try to keep an open mind, but I mostly trust the scientific approaches the most and have not been convinced yet by the evidence for the beyond dreaming stuff (precognitive dreams, shared dreams, other realms, memories of other life times, , though part of me keeps looking into it occasionally just in case, and I am not convinced that none of it is real just have not seen evidence yet that would convince me that any of it is.