Night is the period of ambient darkness from sunset to sunrise during each 24-hour day, when the Sun is below the horizon. The exact time when night begins and ends depends on the location and varies throughout the year, based on factors such as season and latitude. The word can be used in a different sense as the time between bedtime and morning. In common communication, the word night is used as a farewell, mainly when someone is going to sleep or leaving. Astronomical night is the period between astronomical dusk and astronomical dawn when the Sun is between 18 and 90 degrees below the horizon and does not illuminate the sky.