Marc Hébert

in Seasonal Affective Disorder

Second edition

Published on behalf of Oxford University Press

Published in print October 2009 | ISBN: 9780199544288
Published online February 2013 | e-ISBN: 9780191754593 | DOI:

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It is clear that the photoperiod has an impact on light exposure, but this alone cannot account for the disorder inasmuch as the pattern of light exposure is similar for both patients and healthy controls. This suggests a vulnerability, which can appear in some people when moving to higher latitudes. At the moment, there is no evidence of photoperiodic change in healthy controls at the circadian clock or retinal sensitivity level. Therefore, although humans may have retained the capacity to adjust their rhythms with the photoperiod, they do not appear to be photoperiodic anymore. This may be explained by the fact that in our modern society, artificial light may be sufficient to maintain a constant photoperiod. It is clear, however, that although the photoperiod alone cannot account for the disorder, it may represent a triggering factor in some individuals more sensitive to drastic change in light exposure. Even though the origin of such vulnerability is not known at the moment, further research should be oriented towards neurotransmitters (such as serotonin) that appear to be implicated in modulating retinal function, circadian rhythms, and mood.

Chapter.  3199 words. 

Subjects: Psychiatry

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