y favorite time of the year is coming - and it's not Christmas. It's the winter solstice, the shortest day of the year, and in Boston at least, it will arrive on Wednesday at 2:44 a.m.
That's the moment when the sun, at least as seen from the earth, appears to halt in its annual southward trajectory and begins to make its way northward again.
Blame it on my inner pagan. But even if I sleep through the solstice itself, I love it when short days begin to be history and, no matter what storms lie ahead, metaphorical or otherwise, the sun begins to make its annual comeback.
No wonder mystics celebrate. No wonder winter holidays are festivals of light. The sun is not dead.
In terms of understanding the power of sunlight to affect wellbeing, the pagans with their solstice celebrations have long been ahead of the scientists, but the gap has been closing.
Fifteen years ago, Dr. Norman Rosenthal, a South African psychiatrist who grew to hate the dark American winters when he went to work at the National Institutes of Mental Health in Maryland, described a syndrome that millions of sunlight-starved Americans now believe they have in the winter.
It's called SAD, for seasonal affective disorder, and its hallmarks are depression, lack of energy, sleeping too much, eating too much, and gaining weight. Symptoms begin in the fall or winter and often last until spring, when the days get progressively longer - until the summer solstice in June, when the cycle starts over again.
An estimated 10 million Americans have severe SAD and three times that many have milder forms. Not surprisingly, the condition's prevalence increases in places farthest from the equator (where winter days are shortest). One 1989 study showed that SAD affects 1.5 percent of Floridians, but 9 percent of people in New Hampshire.
It would, of course, follow that the remedy for lack of light is light, either the sun itself or the next best thing: ''light boxes'' containing fluorescent bulbs that emit 10,000 lux, a measure of intensity. (That's a lot more than the light in a typical room in a home - 300 to 500 lux - and more than in most businesses, too.)
Light boxes aren't cheap - they cost $250 to $400 or more - but some insurers will reimburse part of the cost. Many companies sell them on the Internet - just search for ''light therapy and depression'' or under the name of a particular manufacturer, such as www.sunboxco.com. You can also check out sites such as www.cet.org/cet2000 (the Center for Environmental Therapeutics) or www.sltbr.org (the Society for Light Treatment and Biological Rhythms.)
Light boxes come in various sizes, from less than two feet by two feet to three feet or so on a side. They can be put on a table top or mounted on a stand. After that, all you do is plop down in front of one and read or watch TV for half an hour or more a day.
Scientists aren't sure why light helps boost mood, though some suspect the secret lies with melatonin, a hormone made by the pineal gland, a pea-sized gland tucked in folds just underneath the brain.
In the evening, the fading of light triggers a chain reaction of nerve signals in the eye and the SCN, or suprachiasmatic nucleus, the spot where the optic nerves cross. These signals tell the pineal gland to start making melatonin, which some data suggest can trigger sleep. In the morning, increasing light shuts down the system and melatonin production falls.
The longer the period of darkness - like the long winter nights in higher latitudes - the more melatonin is secreted; the shorter the night, the less melatonin is made.
Researchers have found that in some people with SAD, melatonin production begins too late in the day. Others have found that people who get SAD show a bigger summer-winter difference in duration of melatonin secretion than those who don't have the syndrome. This suggests, says Rosenthal of NIMH, that people with SAD are more sensitive to seasonal changes in light.
In evolutionary terms, melatonin is an ancient hormone. Over the eons, it may have evolved as a signal to tell animals when to breed. In sheep, for instance, as melatonin levels rise in the fall, ewes become fertile; they then give birth four months later, in the balmier spring.
In human beings, too, light suppresses melatonin production. Light therapy may help SAD patients by re-setting the circadian clock so that sleep-wake up cycles, which drift later and later in winter, get back in sync with other body rhythms, such as hormone secretion, scientists theorize.
In fact, in SAD patients, the problem seems to be an ''internal de-synchrony between sleep and other circadian rhythms - it's like having jet lag for five months,'' says Dr. Al Lewy, vice chair of psychiatry at Oregon Health Sciences University in Portland.
However light therapy may work, dozens of studies indicate that it clearly does help in SAD, says Dr. Dan Oren, a psychiatrist at the Veterans Affairs Medical Center in West Haven, Conn.
In fact, Michael Terman, a psychologist at Columbia Presbyterian Medical Center in New York, believes light is more effective for SAD than antidepressants like Prozac and Zoloft.
Last year, three carefully done studies in the Archives of General Psychiatry supported the case for light therapy. In one, Terman studied 124 people with SAD and randomly assigned them to be exposed to light in the morning or evening.
Twice as many SAD sufferers got better with morning than with evening light, perhaps because it's a more potent way to re-set the circadian clock. In fact, the people with SAD who showed the biggest shifts in body rhythms - as gauged by the time in the evening that melatonin levels began to rise - experienced the biggest antidepressant effects, he notes.
In separate research, Lewy of Oregon studied 51 people with SAD and 49 people without it, exposing them to bright light in the morning or evening for two weeks, then to the opposite schedule. Morning light was more effective, he says.
In the third study, researchers from Rush-Presbyterian-St. Luke's Medical Center in Chicago exposed 96 SAD patients to light in the morning or evening. Morning light was the best antidepressant, though it took several weeks to kick in.
As a practical matter, the message from all this is simple: If you think you have SAD, a light box may help. ''I used to be very conservative and say, `See a doctor [first]','' says Lewy. But ''so many thousands'' of people have used light boxes safely, he says, that that's not necessary. (It goes without saying, however, that if your depression is severe, if you feel suicidal or if there's no seasonal pattern to your depression, you should see a doctor.)
As a scientific matter, though, the data on light and depression are puzzling, in part because there's no obvious link between light deprivation and a deficiency in the brain chemical serotonin, which is believed to be at least partially responsible for nonseasonal depression.
In fact, there's no clear link between abnormal production of melatonin and low levels of serotonin. If scientists could establish such a link, that might be a way to unify the theories of SAD and nonseasonal depression.
There's some preliminary data suggesting that tiny doses of melatonin - 50 to 100 micrograms - taken in mid-afternoon may relieve SAD, presumably by re-setting the circadian clock. But Rosenthal of NIMH cautions that so far, there's too little evidence for people to self-treat this way with melatonin - and it may make some SAD patients worse.
Which leaves light as the best, albeit not always convenient, treatment of choice. If you can manage it and it's sunny, go outside for 15 minutes to 30 minutes as soon as you wake up, though people with SAD often find it hard to drag themselves out of bed to do this. If you can't or if it's cloudy a lot where you live, consider getting a light box. (So far, there's little evidence that light visors or ''dawn simulators,'' which gradually increase light in the bedroom, work as well as the boxes.)
Light boxes emit little ultraviolet radiation, so scientists say there should be no danger to the skin or eyes, though if you're worried, check with your ophthalmologist. You don't need to stare directly at the light - many people read or watch TV.
Or, you can do the pagan thing: Rejoice in the solstice and hunker down and wait for spring.
Judy Foreman is a member of the Globe Staff. Her e-mail address is: foreman@globe.com.
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