Tosefta do Sędziów 7:19

Rebbi says, “There are four watches in a night. [There is a unit of time called] time period (Onah) which is 1/24th of an hour. And [there is another unit of time called] the time (Et) which is 1/24th of the time period (Onah). And [there is a third unit of time called] the moment (Regah) which is 1/24th of the time (Et).” Rebbi Natan says, “There are three watches in a night, as it is said ‘… the beginning of the middle watch.’ (Judges 7:19) There is no [such thing as a] middle, but only when there is something before it and after it.”