Melatonin Suppression by Light in Humans Is More Sensitive Than Previously Reported

J Biol Rhythms. 2015 Aug;30(4):351-4. doi: 10.1177/0748730415585413. Epub 2015 May 27.


The retina drives various non-image-forming photoresponses, including circadian photoentrainment and pupil constriction. Previous investigators showed that in humans, photic suppression of the clock-controlled hormone melatonin is most sensitive to 460-nm blue light, with a threshold of ~12 log photons cm(-2) s(-1). This threshold is surprising because non-image-forming vision is mediated by intrinsically photosensitive retinal ganglion cells, which receive rod-driven synaptic input and can respond to light levels as low as ~7 log photons cm(-2) s(-1). Using a protocol that enhances data precision, we have found the threshold for human melatonin suppression to be ~10 log photons cm(-2) s(-1) at 460 nm. This finding has far-reaching implications since there is mounting evidence that nocturnal activation of the circadian system can be harmful.

Keywords: circadian photoentrainment; human; ipRGC; light; melatonin; pineal; retinal ganglion cell; threshold.

Publication types

  • Letter
  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Circadian Rhythm / physiology*
  • Humans
  • Light / adverse effects*
  • Melatonin / metabolism*
  • Photic Stimulation
  • Pupil / physiology
  • Retina / physiology
  • Retinal Ganglion Cells / physiology
  • Rod Opsins / physiology
  • Vision, Ocular / physiology


  • Rod Opsins
  • Melatonin