"[H]igher than normal levels of norepinephrine during REM are linked to high risk for PTSD." #readingToday
It turns out that higher than normal levels of norepinephrine during REM are linked to high risk for PTSD. This fits into the overnight therapy idea like a perfect puzzle piece since having more norepinephrine in the system means that unconscious bodily responses to emotion (such as a faster pulse or dilated pupils) are not reduced. Abnormally high norepinephrine in REM could therefore prevent the decoupling of emotional content from memories when they are reactivated.
(Moon+ Reader Pro v2.3.3, The Secret World of Sleep: The Surprising Science of the Mind at Rest (MacSci))