The sages of the Mishna (Midot 1:2) provide a vivid description of the nightly routine in the Holy Temple with regard to these watches, and the manner in which they were rendered.
"Each night, the supervisor of all the watches patrols the Temple Mount, inspecting each and every watch. Burning torches are borne before him (in order for the guards to recognize that it is he), and if perchance he encounters a watchman who does not rise up before him, the supervisor cries out to him: "Peace be unto you!" But if the supervisor receives no reply, it is obvious that the guard has been caught asleep. He would then rap the sleeping levite with his stick; the supervisor was even empowered to set his covering on fire (as a punishment for not carrying out his duty properly).
Everyone within earshot who heard the cries of the unfortunate sleeper would say what is that noise in the court? Oh, it is the sound of a levite who has been rapped, and his covering set alight, for he has fallen asleep on his watch.'
Rabbi Eliezer ben Yaakov, a contemporary of that era (and the author of Tractate Midot), related that once, my mother's brother was found sleeping, and his covering was singed'."