Can you let a spirit possess your physical body while you are sleeping? I would like to do this and have a soul- braid with another spirit.
No, because you don’t actually leave your body while you sleep unless you have trained yourself to.
Many people say sleeping is unconscious astral projection
It’s not. Sleep is nothing but a subconscious purging of day to day events. While some people might naturally astral project from the Delta state, that doesn’t mean everybody does. I think whoever claims that is confusing the dream state with the projection state. They have similarities but they are not the same.
You can’t truly give up your body. You reincarnated into it the body is yours. Possession most likely won’t go to a point where it can 100% control your body. I doubt it’ll want to be stuck in a body it doesn’t own in the first place.
That and letting a random entity possess you is in and of itself a foolish newbie desire because you don’t know if the energies are compatible and how it’ll affect your energy system.