Meditating while lying down is one of the 4 basic postures of meditation. Meditating while lying down is the best way to explore the immaterial domains while-out-of-body.
Prior to leaving the body one will often experience a stage of intense energy. This energy has been called 'kundalini,' 'shakti,' 'Shekhinah,' 'holy spirit,' etc.
When this energy arises it is often a profound experience. This profound experience can be frightening to some people. This is why spiritual teachers exist, because a genuine spiritual teacher will have had this experience and can sooth the concerns of a contemplative who has found this experience.
I experience this intense energy in meditation daily. I find when I surrender to this energy, then it takes me into the immaterial domains. So, let go and allow the energy do whatever it wants to.
If you find that your posture needs to be adjusted, say uncross your legs, then follow the dictates of the 'energy,' holy spirit, and do so. If we follow what the holy spirit directs us to do, then we will find that the experience will deepen, and even our mental and physical health will be improved.
Different postures are for different people, because "different strokes are for different folks." Sidhartha Gautama lay upon his side to meditate. Shiva lay prone on his back. Mohammed lay in meditation as well. Some prefer to meditate sitting up in cross-legged position. Others prefer meditate walking, and others prefer to meditate standing. These are the 4 basic meditation postures.
Find the posture that works for you and stick with it. Or, perhaps you will find meditation in one posture for a while, then at another time one of the other postures works for you. I find I meditate in all 4 postures throughout each day. And, when the holy spirit directs me into a different posture I then move into it, because I submit my will to the will of the holy spirit.