Cat water fountains don't use a lot of electricity, and are safe to leave running 24/7. Because your pet may very well drink during the night, it's best to leave the fountain plugged in and running overnight. However, be sure that the fountain has enough water to keep the pump submerged, and that the water looks clean. If necessary, add more water to the fountain before going to bed for the night, or discard old water and refill the fountain with fresh, clean water.