Healthy talk.

From now until April, it's difficult for many of us to maintain our sunny disposition. When your winter blues become extreme, however, you might have Seasonal Affective Disorder (SAD), a condition caused by sunlight deprivation that plagues up to 35 million Americans each year. Interestingly, SAD hits only 1 percent of the population in Florida, while in New Hampshire the estimates hover at 10 percent.

What are the symptoms?

SAD causes mood changes when summer turns to fall. Energy lessens and you may want to eat and sleep more than usual. A bad mood lingers, and you may withdraw socially.

What causes SAD?

During fall and winter, less light gets to the nerve centers that regulate your biological rhythm. This causes changes in sleep and eating patterns, and your body's levels of melatonin and serotonin, substances recognized as sleep and mood stimulators.

Is there a cure?

The most important thing is to bring more light into your life. Spend more time outdoors and sit often in front of bright windows. Purchase a light box, a powerful lamp that simulates sunlight, and sit in front of it 30 minutes each day. For the most benefit, your light box should measure 2,500 to 10,000 "lux," five to 20 times brighter than average indoor light.

To reset your internal clock, go to bed and wake up around the same times every day. Exercise, good nutrition and antidepressants also can help.

Andrew J. Vosburgh, M.D., is medical director for the St. John Health System Occupational Health Partners. For more information about SAD, call [phone].