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I'm reading this Wikipedia article on light therapy and noticed a peculiar statement:

The production of the hormone melatonin, a sleep regulator, is inhibited by light and permitted by darkness as registered by photosensitive ganglion cells in the retina. To some degree, the reverse is true for serotonin, which has been linked to mood disorders.

If I read this correctly, when melatonin is suppressed, serotonin is released? This makes some sense. I'm interested if there is any scientific backing to serotonin being affected by light levels.

Alex Stone
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I'm not sure about humans, but this is true of many vertebrate's retinas. There is experimental evidence showing light-inhibited production of serotonin. This article reviews how serotonin is regulated in response to day lenght (as well as fasting/feeding cycles). The authors suggest a mechanism in which serotonin and ghrelin integrate signals from fasting/feeding and night/day together.

  1. Nowak JZ, Z Urawska E, Zawilska J. Melatonin and its generating system in vertebrate retina: circadian rhythm, effect of environmental lighting and interaction with dopamine. Neurochem Int. 1989;14(4):397-406.

  2. Kirsz K, Zieba DA. A review on the effect of the photoperiod and melatonin on interactions between ghrelin and serotonin. Gen Comp Endocrinol. 2012 Nov 1;179(2):248-53. doi: 10.1016/j.ygcen.2012.08.025. Epub 2012 Sep 5.