Once you have access to spiritual light and sound energy following initiation then meditation must be done sitting upright. For first and second initiates the light energy cannot enter your crown chakra if you are lying down. Upon enlightenment your crown chakra is more open and even when lying down some light may enter however it is much easier when you are sitting upright than lying down.
There are other energy reasons concerning alignment of the chakras and other factors and for all these reasons the best advice is you need to be sitting upright to meditate. It is important to know that the process of gaining access to spiritual light and sound energy is always done sitting upright.