How does melatonin relate to circadian rhythm disorder?

Melatonin: Melatonin feeds the system in regards to pineal glad. Melatonin is the "anti light hormone" so when our body should be preparing for sleep at night, melatonin is secreted telling us to prepare for sleep. What it is not is a sleep aid. Taking supplemental melatonin can be helpful if we are relatively difficient or are trying to iatrogenically adjust our rhythm (shift work, jet lag, etc).

Light/dark night/day:  melatonin is hypothesized to work by regulating the sleep/wake rhythm (by acting on melatonin receptors in the hypothalamus. When it is dark at night, melatonin is secreted which in turn makes the brain ready for sleep. http://www.ncbi.nlm.nih.gov/pubmed/19326288.