In general it's a bad idea to meditate while lying down. Your body becomes very relaxed and it causes your mind to go into "sleep" mode. It will start wandering and you will get more and more unfocused, which is the opposite of the intended effect.

By sitting with good posture (not rigid, just well-aligned) you will stay awake and you will find your focus sharpening rather than wandering all over the place. Meditation while lying down is possible but for a beginner it's a tremendous handicap--a total waste of time in my opinion.