Melatonin administration can entrain the free-running circadian system of blind subjects.

@article{Lockley2000MelatoninAC,
  title={Melatonin administration can entrain the free-running circadian system of blind subjects.},
  author={Steven W. Lockley and Debra J. Skene and K James and Kavita Thapan and J. W. Wright and Jos{\'e}phine Arendt},
  journal={The Journal of endocrinology},
  year={2000},
  volume={164 1},
  pages={
          R1-6
        }
}
Although melatonin treatment has been shown to phase shift human circadian rhythms, it still remains ambiguous as to whether exogenous melatonin can entrain a free-running circadian system. We have studied seven blind male subjects with no light perception who exhibited free-running urinary 6-sulphatoxymelatonin (aMT6s) and cortisol rhythms. In a single-blind design, five subjects received placebo or 5 mg melatonin p.o. daily at 2100 h for a full circadian cycle (35-71 days). The remaining two… 

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