DhyAna Lakshana (sign of meditation) is mentioned very briefly in the following verse by Lord Shiva:
yathA nimilane kAle prapancha naiva pasyati | Tathaiva
unmilanehapi syAdetad dhyAnasya lakshanam ||
Just like the person, who has his eyes closed, can not see the
outer world, in a similar manner, a person who is engrossed in meditation,
It is not recommended that you meditate while lying down. One has to sit at least in an upright position.
Quoting from the book "Meditation and its Practice" by Swami Rama.
Suggestions for Making Sitting Positions More Comfortable:
It is not recommended that you meditate while lying down. There are several reasons for this. One of the most important ...
Meditation is the very delicate art of doing nothing, it is the perfect relaxation. Any effort you put in will get you out of meditation. Its like sleeping, as long as you are asleep you do not know, but as soon as you get aware that you are sleeping, you are awake.
It is best not to analyze the experience or share the experience with others, a good(deep) ...