It is generally safe to leave lights on a Christmas tree, but it is important to use lights that are specifically designed for use on a tree. Using lights that are not specifically designed for trees can be a fire hazard and can cause electrical damage. Additionally, it is important to make sure that the lights are not left on for extended periods of time, as this can cause the lights to overheat and pose a fire risk. When using lights on a Christmas tree, it is best to follow the manufacturer's instructions and to use caution when handling the lights.