The term 'Natural Alarm Clock' is a reference to the phenomenon of DAWN, which is the time of day when the sky begins to lighten as the sun rises. DAWN typically occurs between 4 and 6 am, depending on the season and location. The term 'DAWN' is derived from the Old English word 'dagian, ' which means 'to become day.' As such, it is an appropriate solution for the crossword clue, as it is a natural alarm clock that signals the start of the day.