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.

Does postpartum depression serve some evolutionary purpose?

—Clint Johnson,

Ridgecrest, Calif.

Anthropologist Edward H. Hagen of Washington State University replies:

POSTPARTUM DEPRESSION (PPD), which afflicts 10 to 15 percent of new mothers, may have evolved as a strategic response to a lack of social support because it helped in passing on genes successfully. Many doctors believe PPD is triggered by the changes in a mother’s hormones after giving birth, yet studies have failed to find much evidence for a link between extreme hormone fluctuations and PPD. The fact that fathers, who do not experience such changes, also suffer from PPD is strong evidence that it is not “just hormones.”

The finding that PPD often plagues people who have marital problems or little outside support led biologists Randy Thornhill and F. Bryant Furlow of the University of New Mexico and me independently to propose that PPD has an evolved function. Many animals improve

their chances of passing on their genes if they desert their young when food or parenting help is scarce and invest instead in future offspring that are

more likely to survive and reproduce.

This “parental investment theory” should apply especially well to humans. Human children are “expensive” to raise, requiring years of parenting before they can survive on their own. When a mother lacks support from the father or other family members, she may unconsciously conclude she cannot successfully raise her infant. The ensuing emotional pain from PPD operates somewhat like physical pain: stop what you’re doing—it’s harming your reproductive fitness! Studies confirm that mothers with PPD do significantly reduce parenting efforts and often have thoughts of harming their baby.

This “psychic pain hypothesis” cannot explain the whole story, however, because few parents suffering from PPD abandon their newborn. I propose an additional function of PPD that is like a labor strike, in which a mother’s reduced interest in her baby may serve to elicit help from others. Studies do suggest that higher levels of PPD symptoms in mothers motivate more child care by fathers, and increased social support is one of the

best predictors for the remission of PPD. These hypothesized functions for PPD are far from proved. If you or a loved one is suffering from PPD, contact a doctor immediately—treatments, including antidepressants and talk therapy, are available and effective.