Ask the Brains; April/May/June 2009; Scientific American Mind; by Russell G. Foster, Edward H. Hagen; 1 Page(s)

Because blind people retain a newly discovered system of light-detecting cells, they, too, can suffer from seasonal affective disorder (SAD). Patients who have SAD struggle with serious mood changes in the fall and winter seasons. Symptoms include excessive sleepiness, low energy, and a tendency to crave sweets and starchy foods.

Normally our circadian rhythm is synchronized to the light/dark cycle, but in the absence of such cues our internal physiology starts to drift. The body clock of SAD sufferers may lose synchronization under the shorter periods and lower levels of winter light. Exposure to one to two hours of bright light in the morning often can help correct this disruption and alleviate SAD symptoms. A link between the occurrence of cataracts-clouding in the eye that leads to visual loss-and the development of SAD further suggests that light detection by the eye is key in this disorder.