Out-of-Body Experience
First, leaving the body as a term is not correct. You are never in your body and you are always in your body. This is the paradox that is difficult for the physical mind to explain. Imagine your being as an entity much larger than your physical being/existence. This entity can affect your physical body. Your physical body can react to this entity, they are in constant interaction.
When you talked about the so-called leaving the body, all that happens is that this kind of back-and-forth effect on the physical body weakens. Your focus simply shifts away from the physical body. It's very similar to lying in bed and focusing on relaxing your feet as much as possible. If you start to focus on it, you will feel the different forces and effects acting on your feet much better. As soon as you decide that you want to focus on your thighs instead of your feet, your feet seem to cease to exist, especially compared to the previous state. As you focus on your thighs, you'll feel their interactions better, as well as being aware that your feet are still in place.
This kind of focus jump is what you call leaving the body. If we really want to explain this term, we were talking at most about your focus leaving your physical body and transferring to your energy or astral body.
If you are aware of this, you can perform mental exercises that facilitate this process much more easily. You can hear many different methods, you can get to know them from different sources, but if you dig deep into them, you will notice that the common point in all of them is that they try to shift your focal point.
When you fall asleep during practice, your focal point is as if instead of your feet, it went not to your thighs - as you intended - but to your shoulders, for example. So your focus positioning was not accurate. This is mainly due to habit, as you are used to completely letting go of control over your focus when you go to bed.
Playing with focus is not easy, but it can be learned in the same way that children learn to pay attention to the teacher at school. I can recommend that you persistently and consciously control this focal point even when you fall asleep, when you practice, and also during the day, when you need to keep your focus on a certain person or topic among the many sources of information around you. If you become a master of it, you will realize that these two things are similar in many details.
Let this be today's lesson.

