Check out the notes at Mediumship Development under the
Mystic Academy for some information. What you are experiencing is the natural opening and awareness of mediumship abilities.
You can use meditation in a more conscious state to eliminate what is happening in your sleep state. In that way you will be alert to learn what is necessary without disturbing your sleep.
It is sort of like redesigning your schedule so that sleep is for your rest and the other activities are for your alert time in consciousness. You may need to do some strong inner work to keep yourself in that place of comfort.
BTW the astral is really the lowest level of uneducated and undeveloped spirit existence. Why anyone would want to go there is beyond me. But halting any progress in that direction would be a good start for you. You would find so much more in meditation at a higher level if you stopped this practice.