Hardy trees such as junipers require dormancy. What you are doing is interfering with the natural induction of the winter life cycle. Being outside in the sun during daytime and indoors - assuming a dry 65/70 degree home environment - at night time - would be like, well, Southern Mexico. (bad example?) Japanese Junipers will not grow there. Other junipers might.
Point is that for proper dormancy most junipers need to freeze and go to sleep for the duration of winter. St. Louis weather should not hurt your tree, but now you will need to acclimate it to the cold more slowly. A good idea would be to place it in an unheated garage or shed rather than outdoors. While in dormancy it will not need the sun. When snow is available place some on the soil and around the pot. Check it from time to time but basically leave it alone for the winter. It needs its sleep.
Learn all you can about dormancy.