Melatonin doesn't just improve sleep, but has a lot of neuroprotective effects. They say it may be that P does not reduce ability to produce melatonin, but PD decreases the number of melatonin receptors.
Melatonin helps mitochondria complex 1
Here's another article on melatonin, but also has a list of chemicals that work in animal models of PD. This list aimed at better mitochondrial function, but also most of them have other effects like iron-chelating, antioxidant, and preventing protein misfolding.