If you want to study "the connection between the physical and the astral", start with what you already know and do, because that's a huge hint right there. When you go to sleep, what do you think about? What are you imagining? How do you feel? Etc.

Before asking big questions like "what is physical reality and what are the astral planes exactly?" it's best to clear your mind first of anything distracting you. The last thing you want after putting time in to think about something as clearly as possible is to come up short with rashness getting in the way.

The pedantic has returned.