Exposing the bars to low temperatures or placing them in the fridge will make the bars become firmer. Placing them in the freezer will make them freeze. However, this does not affect the integrity of the bar.
When exposed to high temperatures, the bars will not melt (except the dark chocolate in the Peanut Butter and Almond Chocolate flavors).
In order to preserve the quality of the ingredients, however, we recommend storing your Nii Bars at/below room temperature.