Aside from the Vigiles (basically, firemen and police) mentioned by sempaiscuba in his comment, there were quite a few groups of people who would have been active at night in Imperial Rome.
Perhaps most numerous were household slaves. They could be required to perform a wide range of duties at night as most were expected to be available at all times:
- running errands (e.g. carrying messages)
- if the master went out, slaves would be required to light the way with torches or provide protection
- serving at banquets and generally being on hand to fetch things, fan the guests or assist those who had drunk too much
Pompeii: "painting of banqueting scene from west wall of triclinium. End of the Banquet. The guests are rising but one of them cannot stand up without assistance3." Source: Pompeii in Pictures
- bedroom slaves were in or near the master / mistress’ bedroom at night (even when couples were making out) in case he/she needed anything
"Fresco of couple in bed. Man talks to his shy bride. A servant on the left watches the scene." Source: Wikipedia.
- watchmen and doormen
- nurses for infants
- Weaving and yarning were done by the female slaves of less wealthy Romans who often had their servants multi-task.
Among other groups active at night were:
- Temple attendants
- Workers in taverns and brothels
- Street vendors
- Soldiers acting as sentries for armies camped for the night
- Bakers had to get up early to prepare bread
- Wagon drivers delivering goods
- Entertainers, especially musicians
- Christians sometimes met at night
- Some authors would write at night; this was known as lucubratio. This could have been simply
working late, or waking up and working and then going back to bed.
This could also vary with the seasons, as was the case with both
Pliny the Elder and Pliny the Younger
- According to S. Sticka, "Juvenal relates the importance of fathers training their sons to wake in the middle of the night in order to accomplish their tasks."
- Some slaves also turned to crime at night (as suggested by @bof in
As Jason Linn points out in his doctoral thesis, The Dark Side of Rome (2014) many of these jobs were boring
Not only did many night jobs entail periods of waiting, but often
waiting alone: guards waited for trouble; nurses waited on sleeping
children; prostitutes waited for customers.... With plenty of free
time many night workers had to stay awake and endure boredom alone...
Some guards were even chained to their posts.
Shaun Sticka Segmented Sleep in First-Century Roman Society (Master's thesis, 2017)
K. D. Matthews, The Embattled Driver in Ancient Rome
A. R. Ekirch, At Day's Close