פירוש על ברכות 5:5

The division of the day was reckoned as follows : From sunrise to sunset consisted of twelve "hours," which would be longer in Summer and shorter in Winter. The sixth hour ended at noon ; hence it is convenient to speak of the day as commencing at 6 a.m., and the night (which was similarly divided into twelve hours) at 6 p.m. "Until the end of the fourth hour" would accordingly be 10 p.m.

The Romans divided the night into four vigiliae. So also Matt. xiv. 25, Mark vi. 48.