Curfew!

Of course people couldn’t instantly get home. It’s likely the 6 pm signal was a first warning, to give you time to pack up and find your way home - which could be quite far from where you worked. A later ‘blast’ was probably sounded around 9 pm, by which time you HAD to be indoors - what we call a curfew. This made it easier for the state (government, police...) to ensure that all was quiet, and the city safe from intruders. As the Aztec empire expanded, more and more enemies were made, so security was a big issue. Book 8 of the Florentine Codex stresses the role of the ruler himself (he can be seen in the picture) in this:-
’He kept careful vigil... He himself sometimes set forth by night, circled the city, and beheld what was done... For the ruler slept not, nor any of the Mexicans’. Well we reckon that’s a bit OTT, but it shows just how seriously security was taken!

Both in the ruler’s palace as in officials’ houses all over the city, fires were kept alight all night, and a small army of youths took turn to keep guard.
’None slept in his [own] home. The task of the youths was to go into the forest, in order that watch be kept within the city; for nightly there was light and fires were burned; thus was guard maintained...’

Source:-
Historia de la Vida Cotidiana en México: 1 Mesoamérica y los Ambitos Indígenas de la Nueva España by Pablo Escalante Gonzalbo, Mexico City, 2004
Image:-
from the Florentine Codex (original in the Biblioteca Medicea Laurenziana, Florence) scanned from our own copy of the Club Internacional del Libro 3-volume facsimile edition, Madrid, 1994.