It depends on the type of generator. A small portable generator may need a break from running after 8 to 12 hours (check your owner’s manual), while a standby generator is designed to run continuously if needed. Operating a portable fuel-powered generator overnight may not be safe unless someone is awake to monitor it. While today's manufacturers take precautions to make their generators as safe as possible, there is always a risk of fire if something flammable gets too near the generator. Additionally, fuel-powered generators produce carbon monoxide, a toxic gas that can be hazardous if inhaled. In addition to not running a generator overnight unmonitored, don't run one in a basement, a garage, or any enclosed space. Always follow the manufacturer's instructions for safe operation.