Who invented candle clocks?

The inventor is not known, but the earliest documented use of a candle clock is from approximately 500 A.D. in China. A Chinese poem by You Jiangu describes a clock that uses six candles that are marked in increments of 20 minutes. Each candle takes four hours to burn.

Candle clocks were also used by King Alfred the Great of England and in medieval churches. Candle clocks were not reliable timekeepers due to variations in the burning rates of candles.

An alarm clock was set up by inserting a nail in the candle, then putting the candle in a metal dish. When the wax burned down, it eventually freed the nail, which clattered into the dish.