Vegetius, writing (probably) in the late fourth century, records that “daily, even in peacetime, soldiers from all centuries and sections in turn perform the night watch, as well as guard or outpost duty, and so that no one is unjustly overburdened or given exemption, the names of those who have done their turn are entered in the notes' (Epitome of Military Science 2.19.3).
Polybius, writing over 500 years earlier in his famous digression on the Roman camp of the Republican era, informs us that “a guard detail consists of four men” (6.33.7). Nowhere does he state why this should be the case, but an explanation is provided by Vegetius: “Since it was understood to be impossible for individuals to remain vigilant while on watch, for that reason the night-watches were divided into four parts using the water-clock, so that it would be necessary to stay awake no longer than three hours at night” ( 3.8.17). Simple arithmetic confirms that
