The actual study also says that those engaging in gun violence get less sleep. It’s not clear what direction the causality is for that situation - are they getting less sleep because they’re awake late engaging in those crimes or because their criminal involvement disturbs rest?

But for the piece about just witnessing violence - I can see that causing less sleep in the same way that victims of burglaries or home invasions or rape experience less sleep. You end up being highly anxious and can’t have restful sleep due to fear of being a victim again.