Dreams

1. The most vivid and "realistic" dreaming occurs in rapid eye movement (REM) sleep, which is concentrated more in the second half of the night. While some dream activity does occur during deep (or slow wave) sleep as well, it is much less formed and structured, and recall of the content of those dreams is much more limited.
2. In REM sleep, there is loss of almost all muscle tone, except for the diaphragm and eye muscles. This explains why it is very common for people to describe feeling paralyzed and unable to move while dreaming of a stressful situation (such as being chased).
3. That being said, there is a rare condition known as REM behavior disorder in which people violently act out their dreams, often to the point of causing injure to themselves and/or their bed partners. This is mostly seen in older men, and can be a harbinger of degenerative neurological disorders, such as Parkinson's disease.