It is not safe to leave a candle burning overnight, as it can cause serious damage to living organisms and properties. Leaving a candle burning for around four hours can cause carbon build-up, creating an unstable flame that can lead to wax tunneling, soot damage, and damaging the candle container. Beyond that, leaving a candle burning overnight raises the risk of other incidents, such as the candle being knocked over, which can potentially cause a house fire. Another potential incident of leaving a candle burning overnight is a candle flashover, which is when all the wax in the candle melts and catches flame of the wick and is ignited, potentially setting fire to anything close to the candle. In addition, burning candles for too long can release toxic substances that are harmful to health, especially the respiratory system. It is safe and advisable to not leave a candle burning unattended, even for a short period of time, and candles should never be burned unsupervised. If you realize you left a candle burning, you should immediately extinguish it and ensure that it is completely out before leaving the room.