Oh, the light! The autumn light! Is there anything more glorious than an October day, awash in the sun's low-slung amber rays?

And yet ... perhaps you feel the dread, too. The looming inkiness that, like the tide, crawls up your legs a little higher each day, turning that honeyed light to molasses and molasses to muck until you realize, too late, that the birds have left and the world has gone dark. Dark when you wake up, dark when you go home.

In simpler times, we slept more in winter, but modern living denies us that luxury. So increasingly each day, soft-white lights from yonder windows break — along with halogens, tungstens and compact fluorescents. And when we can't stand it anymore, we resort to manipulation, declaring that 6 in the morning is now 5.

Now science is finding that our manhandling of light and time is making us sick.

Artificial illumination is fooling the body's biological clock into releasing key wakefulness hormones at the wrong times, contributing to seasonal fatigue and depression. And daylight-saving time, extended by Congress this year for an extra four weeks, risks dragging even more Americans into a winter funk.

Much more than mental health is at stake. Women who work at night, out of sync with the light, have recently been shown to have higher rates of breast cancer — so much so that an arm of the World Health Organization will announce in December that it is classifying shift work as a "probable carcinogen."

That will put the night shift in the same health-risk category as exposure to such toxic chemicals as trichloroethylene, vinyl chloride and polychlorinated biphenyls, or PCBs.

"Electric lights are wonderful, but as with a lot of other things, we really mess

things up," said David Avery, a psychiatrist at the University of Washington School of Medicine who studies light's impact on health. "Our ancestors evolved in a very regular light-dark cycle, and our bodies just work better that way. But more and more, we are creating very irregular, erratic lighting cues."

Regulating melatonin's ebb and flow

Researchers have long known that virtually all living organisms have biological rhythms that are linked to light. But the human health implications remained opaque until the 1970s, when scientists discovered the brain's internal clock: the suprachiasmatic nucleus, or SCN, a tangle of neurons in the hypothalamus connected directly to the eyes.

The SCN controls the ebb and flow of hormones that influence sleepiness, alertness and hunger. Prime among them is melatonin, levels of which rise each evening, easing the onset of sleep, and then fall before dawn in advance of awakening.

The SCN takes its cues from light signals passed along by the eyes.

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 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: Their clocks run fast compared with solar clock time, lulling them to sleep early and then awakening them well before dawn's early light.

Seasonal affective disorder blues

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.

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 — where days are shorter — 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 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."

The good news is that treatments for seasonal depression — primarily the use of bright light, and in some cases melatonin supplements, to reset the body's clock — can be effective.

So effective is light as a mood improver that many psychiatrists now suspect that their understanding of depression has been backward: The disturbed sleep and withdrawal into darkened rooms often seen in patients with depression, bipolar disorder and related problems may be not a symptom of those diseases but a cause. Reset the clock, and the depression lifts.

A 2005 review commissioned by the American Psychiatric Association concluded that daily exposure to bright light was about as effective as antidepressants against several forms of depression.

Subduing insidious night-shift light

That artificial lighting can reset people's clocks and boost alertness at night speaks to its potential to throw normal rhythms into disarray. As though it were not bad enough that lighting is a 24-7 feature of modern life, said Avery of the University of Washington, people spend evenings staring at their "Microsoft blue" computer monitors, then wonder why they can't fall asleep.

"We've deseasonalized ourselves," said Thomas Wehr, a psychiatrist at the National Institute of Mental Health in Bethesda, Md. "We are living in an experiment that is finding out what happens if you expose humans to constant summer-day lengths."

The perfect solution is to give up artificial light, an approach that quickly brings one into a cycle of long, restful nights and easy awakenings at dawn. More realistically, experts recommend avoiding bright lights after dusk and perhaps wearing yellow sunglasses at brightly lit evening events to filter out the blue light that might fool your ganglia into thinking it is morning.

For those working at night, "the idea might be to have a work environment where at the beginning of the shift the lighting is heavier in blues that suppress melatonin, then gradually it changes and becomes redder and redder," a hue that does not stimulate the eye's ganglion cells, said Richard Stevens, an epidemiologist at the University of Connecticut Health Center.

Stevens knows how important night-shift lighting can be. It was his focus on the issue that helped reveal that women who work night shifts for 20 to 30 years have breast cancer rates 30 percent to 80 percent higher than their day-shift counterparts. The mechanism is still not fully explained, but studies have since shown that melatonin — whose secretion is suppressed by nighttime illumination — is a potent anticancer hormone.

A panel of experts convened by the International Agency for Research on Cancer, part of the World Health Organization, concluded there is strong evidence that shift work can cause cancer, leading to the agency's pending declaration that shift work is a probable carcinogen.

The solution for the workplace, and for people struggling to get through the winter, may come in the form of light-emitting diodes, or LEDs, which can be tuned to any color. But until LEDs become more affordable, people who can will for the most part be best off sinking obligingly into the long, gray flannel night and avoiding the midnight lighting they think they crave.

Darkness, however, doesn't have to be about depression and loneliness, said Dave Crawford, executive director of the International Dark-Sky Association, a Tucson-based nonprofit that advocates against unnecessary illumination.

It can be about stars, about contemplation, about quiet conversation with a friend.

"If we sprayed water all over the place here in the desert, we'd be put in jail. So why is it OK to spray light all over the place at night?" asked Crawford.

Light is fine — in the day — Crawford said. "We're trying to bring to everyone's attention that there is a night."

For the next few months, that is going to be hard to forget.