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As winter approaches and daylight hours grow shorter, people prone to seasonal depression can feel it in their bodies and brains.

The millions of people estimated to have seasonal affective disorder, or SAD, typically have episodes of depression that begin in the fall and ease in the spring or summer. One trigger for SAD is when clocks revert to standard time, which happened Sunday across much of the U.S., bringing darker afternoons. A milder form, subsyndromal SAD, is recognized by medical experts, and there's also a summer variety of seasonal depression, though less is known about it.

In 1984, a team led by Dr. Norman Rosenthal, then a researcher at the National Institutes of Health, first described SAD and coined the term. "I believe that because it is easy to remember, the acronym has stuck," he said.

No single cause for seasonal affective disorder has been identified. The disorder affects many more women than men, according to the NIH. Symptoms can be similar to depression and can include persistent feelings of hopelessness or pessimism and difficulty concentrating, remembering or making decisions.

In one effort to better understand what's going on, scientists are learning how specialized cells in our eyes turn the blue wavelength part of the light spectrum into neural signals affecting mood and alertness.

Sunlight is loaded with the blue light, so when the cells absorb it, our brains' alertness centers are activated and we feel more wakeful and possibly even happier.

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