Nychthemeron or nuchthemeron, (Greek: Νυχθημερόν ; nykt-"night", hemera-"day") is a full calendar 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.
The period of time known as the nychthemeron thus defined the Daily Cycle of divine services of prayer and worship that punctuates each liturgical day in the life of the Orthodox Church.
- Prime (liturgy) (First hour).
- Terce (Third hour).
- Sext (Sixth hour).
- None (liturgy) (Ninth hour).
- Vespers (Hespera / Evening).
- Compline (Apodeipnon)