Nychthemeron or nuchthemeron, (Greek: Νυχθημερόν; nykt-"night", hemera-"day") is a full day of twenty-four consecutive hours, night and day. The term is found in the New Testament, in 2 Corinthians 11:25.
Following Roman custom, the Byzantines began their calendrical day (nychthemeron) at midnight with the first hour of day (hemera) coming at dawn. The third hour marked midmorning, the sixth hour noon, and the ninth hour midafternoon. Evening (hespera) began at the 11th hour, and with sunset came the first hour of night (apodeipnon). The interval between sunset and sunrise (nyx) was similarly divided into 12 hours as well as the traditional "watches" (vigiliae) of Roman times.
In addition, the term is sometimes used especially in technical literature, to avoid the ambiguity inherent in the term day. It is the period of time that a calendar normally labels with a date.
- Prime (liturgy) (First hour).
- Terce (Third hour).
- Sext (Sixth hour).
- None (liturgy) (Ninth hour).
- Vespers (Hespera / Evening).
- Compline (Apodeipnon)