henry copeland:

Light boosts serotonin

Exposure to bright light is a second possible approach to increasing serotonin without drugs. Bright light is, of course, a standard treatment for seasonal depression, but a few studies also suggest that it is an effective treatment for nonseasonal depression38 and also reduces depressed mood in women with premenstrual dysphoric disorder39 and in pregnant women suffering from depression.40 The evidence relating these effects to serotonin is indirect. In human postmortem brain, serotonin levels are higher in those who died in summer than in those who died in winter.41 A similar conclusion came from a study on healthy volunteers, in which serotonin synthesis was assessed by measurements of the serotonin metabolite 5-hydroxyindoleacetic acid (5-HIAA) in the venous outflow from the brain.42 There was also a positive correlation between serotonin synthesis and the hours of sunlight on the day the measurements were made, independent of season. In rats, serotonin is highest during the light part of the light–dark cycle, and this state is driven by the photic cycle rather than the circadian rhythm.43,44 The existence of a retinoraphe tract may help explain why, in experimental animals, neuronal firing rates, c-fos expression and the serotonin content in the raphe nuclei are responsive to retinal light exposure.44–48 In humans, there is certainly an interaction between bright light and the serotonin system. The mood-lowering effect of acute tryptophan depletion in healthy women is completely blocked by carrying out the study in bright light (3000 lux) instead of dim light.49
- www.ncbi.nlm.nih.gov