For the past ten years, we've had very low bedside tables. Whether it's a low cut side table or a vintage trunk, I've tended to go low slung. This past weekend, however, we rearranged and moved our IKEA Anes dressers next to our bed to shake things up a bit. What I thought I was getting was a new, clean look, what I didn't suspect was how MUCH MORE COMFORTABLE it was and how big a difference it made that the side table/dresser was a few inches above our bed. Whether we are standing and reaching for the side table to pick up a book or telephone, or laying down in bed and reaching for a glass of water, this new height has been key. When standing, you don't have to stoop and when you're in bed it's easier to reach UP than DOWN. I also find that I don't kick over glasses in the middle of the night like I used to when the table was low to the side (I can be a restless sleeper).
What is the perfect height? I would start with 27" for the side table or nightstand (near table height) and then adjust your bed lower from there. Ours is about 4-5" from mattress to side table, which is perfect for us right now. The main thing is that you can comfortably reach your side table from a standing position (without stooping) and that you can also comfortably reach UP from your bed.