If the unit is turned on, it will continue to run until it is almost dry. The good thing is you do not have to keep a regular watch on your diffuser. This product automatically shuts off when the water has been used up. You can rest your worries about fire hazards in your house. This diffuser is perfectly safe to use. When the water runs out and the unit shuts off, the light remains turned on. It is designed to diffuse essential oils and the same time double as a night lamp.