Seeing the Light of Day

But the SCN does not work in a vacuum. It takes its cues from light signals passed along by the eyes.

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For decades scientists presumed that those clock-setting signals came from rods and cones, the light-sensitive cells in the retina that provide black-and-white and color vision. Then, in 2002, researchers at Brown University discovered an entirely different set of light-detecting cells in the eyes of humans and other mammals: ganglion cells.

Unlike rods and cones, ganglion cells specifically detect sky-blue light. The amount of light needed to get them firing is about 500 billion photons per second per square centimeter, or the intensity of sunlight reaching the eye at about daybreak. Taken together, those traits make them the perfect cells to tell the brain when dawn has arrived, which they do via a dedicated neural conduit to the SCN.

Unfortunately, this system does not always work like clockwork.

Because of genetic differences, many people's clocks are set differently from others'. In some, the evening melatonin spike is delayed and sleep comes late. Early awakening is also often difficult for these night owls, perhaps in part because their melatonin levels have not had time to drop sufficiently by morning.

Others have the opposite problem: The clocks in these morning larks run fast compared with solar clock time, lulling them to sleep early and then awakening them well before dawn's early light.

Being out of phase with the natural day-night cycle can take a big toll, causing fatigue, mood disturbances and depression. But for millions of Americans, these symptoms become even worse in winter, blossoming into what is in effect a months-long case of jet lag.

Scientists disagree on the cause of seasonal affective disorder, or SAD, as it has come to be known. Some focus on winter's late sunrises, which appear to push various hormone cycles out of phase with the daily wake-sleep cycle. Others focus on the early sunsets, which may affect the timing of melatonin production in the brain.

But while genes clearly play a role (night owls are more often affected), location also matters.

Recent work by Thomas White of the New York State Office of Mental Health and Michael Terman, director of the Center for Light Treatment and Biological Rhythms at Columbia University Medical Center, has shown that seasonal depression and mood disorders become more prevalent not only at northern latitudes -- not surprising, as days are shorter there -- but also toward the western edges of time zones, where people remain in darkness almost an hour later each morning than their same-timed counterparts farther east.

Daylight saving time, which has been stretched this year to Nov. 4 for a number of reasons, including an effort to save energy, exacerbates the problem by further delaying the time of sunrise, a key signal that resets the body's clock each day.

"From the psychiatric perspective, the extension of daylight saving time this year was a very bad decision," Terman said. "Our expectation is we will see increased depression and mood disorders."