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Seasonal affective disorder is treated with light therapy, but the mechanism has been poorly understood. To investigate this issue, investigators altered the usual murine cycle of 12 hours of light and 12 hours of dark to a total of 7 hours. Mice with the shortened cycle had similar sleep amount and architecture as 24-hour cyclers.
Compared with 24-hour cyclers, 7-hour cyclers had higher corticosterone levels (the mouse equivalent of human cortisol) and had impaired performance on tests used to assess cognition and depression-like behaviors. Seven-hour cyclers had greater activity, as measured by cells with expressed early gene c-Fos, in hippocampal areas associated with memory ability and in limbic areas (amygdala and habenula). Localizatio…