This study in the Journal of Pineal Research shows that melatonin has a variety of interesting and potentially beneficial effects regarding the modulation of mitochondrial metabolism.

Melatonin protects the mitochondria from oxidative damage reducing oxygen consumption, membrane potential, and superoxide anion production.

J Pineal Res. 2008 Nov 19; PMID: 19054298
Lopez A, Garcia JA, Escames G, Venegas C, Ortiz F, Lopez LC, Acuna-Castroviejo D.

Centro de Investigacion Biomedica, Parque Tecnologico de Ciencias de la Salud, Universidad de Granada, Granada, Spain.

The role of melatonin in improving mitochondrial respiratory chain activity and increasing ATP production in different experimental conditions has been widely reported. To date, however, the mechanism(s) involved are largely unknown. Using high-resolution respirometry, fluorometry and spectrophotometry we studied the effects of melatonin on normal mitochondrial functions. Mitochondria were recovered from mouse liver cells and incubated in vitro with melatonin at concentrations ranging from 1 nm to 1 mm. Melatonin decreased oxygen consumption concomitantly with its concentration, inhibited any increase in oxygen flux in the presence of an excess of ADP, reduced the membrane potential, and consequently inhibited the production of superoxide anion and hydrogen peroxide. At the same time it maintained the efficiency of oxidative phosphorylation and ATP synthesis while increasing the activity of the respiratory complexes (mainly complexes I, III, and IV). The effects of melatonin appeared to be due to its presence within the mitochondria, since kinetic experiments clearly showed its incorporation into these organelles. Our results support the hypothesis that melatonin, together with hormones such as triiodothyronine, participates in the physiological regulation of mitochondrial homeostasis.