A normal resting heart rate is more indicative to your problem being dehydration. Some of us just utilize sodium at a faster rate than others. It can be seen in family histories through kidney stones, migraines, and epilepsy. The latter two usually being on the female side, but males in the family can have them as well. If you went and had your sodium checked, it might come back normal. If you went and got it checked later that afternoon, it might come back low. See if you crave salt more in the afternoon or evening. Low sodium causes you to wake up early at night and usually has your brain racing with thoughts.