Tmd: my jaw is sore by the end of the day, (not clenching). could it still affect me if i clench at night? it's worst when i sit or lie on my back..

Yes: Some joint displacement and pterygoid hypertrophy are the result of chronic nocturnal clenching. These nighttime events create tremendous muscle force and activity that can be at play. Find a dentist trained in treating tmd pain with a nti devise.

Yes: During clenching, the muscles become even more fatigued than they are during the day, so they never rest. Compromised posture may play a role. Neck pain is common. Avoid repetitive behavior like chewing gum. When you lie back, the lack of support causes the jaw to fall back, exacerbating the problem. I would be concerned about sleep apnea. I think there are better treatments than nti appliances.

Grinding: Grinding of the teeth occurs at night or during sleep, clenching during awake hours. The cumulative affect of grinding at night, when you sit or lie on your back, signals that there is some type of an alignment problem (neck, bite, etc.) i would see a dentist/tmj-tmd specialist to have an evaluation of your bite while you are in different positions.