Roman daytimes were called hora (hours), with the morning hour as hora prima. The night was divided into four sections called vigilia (night watch), two before midnight and two after.The Roman Calendar - www.die-roemer-online.de, after: Johannes Irmscher, Renate Johne (eds.): Lexicon of Antiquity. Wilhelm Heyne Verlag, Munich 1962. The Romans originally counted the morning hours backwards: "3 a. m." or "3 hours ante meridiem" meant "three hours before noon", in contrast to the modern meaning "three hours after midnight".
