This is not true. When we "astral travel" we have something called "silver cord", connecting the material body to astral body. Although it's probably you gonna hear/see/feel dark entities, it's just "mental" (if you end up really scare, you don't die, you just wake up). In "astral/realm of dreams/whatever" all thoughs/intentions become true, so your experience depends on what's going on your subconscious mind... If you are not aware of the things that can influence your subconscious mind... Music, Movies, Web, TV... Avoid horror movies (I don't like action movies neither when astral project).

Never heard about "body possession" during sleep... We all dream during sleep (some might not remember much, others might remember almost everything.... some conscious and some unconscious), so "body possession"? Pretty sure that can't happen... The keyword is: "Frequencies", dark beings = low frequencies; angelic beings = high frequencies... For example: a angelic being can be on a low frequency dimension without being noted nor be seen by the dark entities! (only if wanted to)... but it cannot happen otherwise (dark being can not be, nor are aware of the high frequency dimensions). And yes, we have high dimension guides looking for us... So body possession it's something that is hard to believe for me (while sleeping). I just think that you are more susceptible for body possession while drunk (for example) than on sleeping... but each case it's unique, everyday learning.