Melatonin as a Mitochondria-Targeted Antioxidant: one of evolution's best ideas (2017)

It is important to note that while excess ROS (Reactive Oxygen Species; oxidative stress inducers) generation is bad, ROS also activate many signalling pathways in moderate concentrations. A little oxidative stress is required for neurogenesis, angiogenesis, Metformin-induced insulin sensitization, as well as muscle growth.

Melatonin, while known as a potent antioxidant, can also conditionally have pro-oxidant capabilities, as it has in cancer cells. Thus, unlike NAC, which is an absolute unconditional antioxidant, Melatonin seems to decrease ROS when it's in excess, but also increase it when it's lacking.