Are serotonin levels in humans affected by light? 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.**

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.