Are (baseline) high blood levels of melatonin associated with either circadian rhythm or mood disorders? (Exogenous) melatonin is administered at night for treatment of circadian rhythm disorders, but melatonin taken during the day is linked with depression. I would like to know if high blood levels of (endogenous) melatonin (at a steady state, throughout the day) might be linked with mood or sleep disorders (or have any effect at all), or if the body's response to melatonin adjusts accordingly. I think the answer to this SE question is related, but I can't find the source using a reverse image search.

I found a recent PLoS article that said that none of hypochondriasis, depression, hysteria, psychopathic deviate, masculinity-femininity, psychasthenia, schizophrenia, or social intraversion (on the MMPI-2) "were associated with any melatonin parameter."

However, it does appear that "increases scores of Hypomania and Paranoia were associated with a shorter DLMO-DLMOff interval" (duration of melatonin production).

The only thing that concerns me are the significance and the effect size of the latter two results. I can't tell whether they corrected for multiple comparisons or not.

Reference: Burgess HJ, Fogg LF (2008) Individual Differences in the Amount and Timing of Salivary Melatonin Secretion. PLoS ONE 3(8): e3055. doi:10.1371/journal.pone.0003055