A wistful feeling comes over us in late autumn, as the last remaining leaves drop, morning frosts cover the ground, and the sun sets earlier each day. Hot cider and the warmth of a favorite old coat may be all you need to face the coming winter with good cheer, but for many people, fall melancholy deepens to winter depression.

Winter depression is still a mystery to scientists who study it. Many things, including brain chemicals, ions in the air, and genetics seem to be involved. But researchers agree that people who suffer from winter depression -- also known as "seasonal affective disorder," a term that produces the cute acronym SAD -- have one thing in common. They're particularly sensitive to light, or the lack of it.

Many studies have shown that people with seasonal affective disorder feel better after exposure to bright light. It seems simple enough: In higher latitudes, winter days are shorter, so you get less exposure to sunlight. Replace lost sunlight with bright artificial light, and your mood improves. But it's actually far more complex. Alfred Lewy, MD, a seasonal affective disorder researcher at the Oregon Health & Science University, says it's not only a matter of getting light, but also getting it at the right time. "The most important time to get light is in the morning," he says.

He thinks seasonal affective disorder is due to a "phase-shift" of the circadian rhythm. The wall clock may tell you it's time to get up and at 'em, but your body's internal clock says you should be resting. Bright light in the morning resets your circadian clock.

This is relevant to the "fall back" time change, which happens in places that observe Daylight Saving Time. You might think that setting back the clock one hour would make seasonal affective disorder symptoms worse, because the sun sets one hour earlier. "Actually, I think it's the opposite," Lewy says. "The problem is waking up before dawn."

Lewy says he suspects that "true winter depressives," the people whose problem is biological and not related to other factors, might feel better after the time change. But the improvement would only be temporary, as days continue to shorten.