Do blind people ever suffer from seasonal affective disorder? If so, can sunshine or tanning beds help?
—Kirstin Steele, Charleston, S.C.
Circadian and vision neuroscientist Russell G. Foster of the University of Oxford answers:
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.
Puzzlingly, some people who are completely blind—lacking the eye’s photoreceptors known as rods and cones—can experience SAD. A decade ago scientists at Cornell University proposed that humans can detect light through their skin. But when researchers in the Netherlands tested this idea by exposing just the skin of SAD patients to bright light, they found the treatment had no effect at all. How, then, are they detecting light?
In 1999 we found that mice lacking rods and cones were nonetheless able to synchronize their circadian rhythm to the light/dark cycle. These observations led to the discovery of an additional photoreceptor system in the retina of humans and other mammals consisting
of a small number of photosensitive retinal ganglion cells (called pRGCs). These cells are most sensitive to blue light, and, significantly, blue light is most effective in alleviating the symptoms of SAD. We think that blind people can develop SAD because their other photoreceptor system—the pRGCs—remains intact. Likewise, although there are no known studies of light therapy in those who are visually impaired, we suspect light could be used to treat SAD symptoms in blind patients.