It's all about stress and anxiety. You can control it during the day be forcing yourself to relax your mouth and jaw but at night you have no control, which is why Dentists prescribe the mouth guard to wear during sleep, to protect your teeth. You can wear the guard by day if you need to, but it's difficult to speak. It's crucial you stop the process pronto or risk jaw pain and trauma to the joints, which can cause clicking and popping sounds. That is a horrid side effect! I exercise everyday to keep my body relaxed and it helps especially if you exercise in the evening, not too much, just a little. The grinding is normal animal behavior that comes and goes with life's trials, but you need to get a grip on it.