Hi Peter -
"Intruder" dreams are some of the most frightening nighttime experiences we can have. The common theme, in all of them, is a terrifying sense of vulnerability.
You have asked the key question that all people who have these dreams want to know: What is their psychological significance? Is the dreamer recalling a repressed memory from long ago? Was he or she abused as a child? And just who is "this man" (the attacker almost always is male) who attacks or chases us?
All of these concerns are legitimate, but before we busy ourselves with creating a past that may not exist (your "unresolved issues"), it is important to recognize physical factors that more likely are responsible. During each of our REM periods at night (which, as you correctly note, come in 90-minute intervals) a natural paralysis of the body occurs. This paralysis prevents us from actually getting out of bed and "acting out our dreams."
If we become aware of this paralysis during a dream (which is not uncommon), we can experience the sensation that we "can't move," or that we are being "held down" by an alien force. Because we feel vulnerable during "dreams of paralysis," many of us immediately begin dreaming of an attacker, as if in answer to our deepest fears!
As a rule, dreams of intruders are not memories of experiences of being attacked. Rather, they are an almost universal response, experienced by men and women alike, to the vulnerability we feel when we realize our "REM paralysis." And as you point out, these dreams also are highly associated with sleeping on our backs.
What's the solution? You are having a simple run-in with the age-old "phantom terror of the night." (His name, by the way, is REM SLEEP). Why don't you try rolling over, and see if this doesn't help you to dream in peace?
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