Here’s a little thing on posture from Jon Kabat-Zinn. It’s from his book Mindfulness for Beginners. The full excerpt (if there is such a thing) called “Getting Started with Formal Practice” addresses a number of other topics (what to do with your eyes, sleepiness, protect this time).
The carriage of your body during formal practice is important. It helps if you adopt a posture that embodies wakefulness, even or especially if you feel sleepy. That probably means not practicing lying down, although lying down can be a wonderful way to cultivate mindfulness and wakefulness as we do in various body scans and lying-down meditations. If you set your intention at the beginning of a period of practice to “fall awake” instead of “falling asleep,” then it is fine to experiment with practicing lying down.
Aside from the fact that you can also meditate formally when standing still or while walking, a posture that embodies wakefulness usually suggests sitting, and sitting in such a way that the back is straight but relaxed, with the shoulders and arms hanging off the rib cage, the head erect, and the chin slightly tucked. You can sit either on a straight-backed chair or on a cushion on the floor. As best you can, sit in a posture that naturally and easily embodies dignity and presence for you.