It could be normal for you. It could also be a sign that you are tending to sleep with your mouth open. Worst case scenario, it could be indicating that you have a thyroid problem.

If you are finding that you wake up often with your mouth open, you may need to start temping vaginally. I would also find (and particularly important in colder weather that we are now heading into) that putting the cold thermometer in your mouth and temping straight away was a recipe for disaster. I found it best to put the thermometer in my mouth, wait for it to stop feeling cold, then move it to the other side of my mouth (keeping my mouth closed the whole time) and only then turn it on.

BW