However, when we're compressed, we strain against gravity to keep ourselves up. That muscular tension translates into mental tension. Also, if the muscles tire, and the torso and head fall into a slumped position, breathing becomes more superficial. When there is less oxygen circulating, the body and mind become sluggish. How you hold your head, eyes, jaw, mouth, and arms is also important.
Different traditions stipulate different positions.
However, you can explore on your own what is most effective to avoid dreaminess and foster strong concentration. Notice your head's position. Compare what happens when you keep your eyes fully or slightly closed with when they're wide open. Remember that the right posture for you is not necessarily what you see someone else doing, no matter how good it looks. And what's right for you in the first six months of meditating may not be right for you after six years, because the body is changing all the time.