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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:
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
"Fresco of couple in bed. Man talks to his shy bride. A servant on the left watches the scene." Source: Wikipedia.
Among other groups active at night were:
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.
Other sources:
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