In this paper, we investigated how "lying down'' body postures affected the use of the smartphone user interface (UI) design. Extending previous research that studied body postures, handgrips, and the movement of the smartphone. We have done this in three steps; (1) An online survey that examined what type of lying down postures, participants, utilized when operating a smartphone; (2) We broke down these lying down postures in terms of body angle (i.e., users facing down, facing up, and on their side) and body support; (3) We conducted an experiment questioning the effects that these body angles and body supports had on the participants' handgrips. What we found was that the smartphone moves the most (is the most unstable) in the "facing up (with support)'' condition. Additionally, we discovered that the participants preferred body posture was those that produced the least amount of motion (more stability) with their smartphones.