I've fallen into this habit of laying in my bed playing guitar. 1. My neck strains because I'm usually having to peek at the fretboard. 2. I'm noticing numbness in my hands and wrists. 3. It affects my technique negatively. I can't pick lying on my back so I use my fingers. I need to practice with a pick. Bass sometimes gives me bad trigger finger (pinky) if I really dig in and am jamming for hours. Bass is more physical but I didn't feel like I was getting carpel-tunnel when I played. I set up a practice space with a comfortable chair (the one in my avatar) and as soon as I was done I lay down on my bed and played my guitar as if on auto-pilot. Feel free to post your bad habits. I might have habits that I don't even know are bad yet. I don't know what I don't know.