Come January, when the euphoria of the holidays has faded and the days are short, a sense of melancholy is pervasive among northerners. Seasonal Affective Disorder (SAD), commonly referred to as the "winter blues," is marked by feelings of lethargy, depression, lack of appetite and lack of desire to have sex.

It may surprise you to learn that sunlight deprivation can cause all of these symptoms and more. Though not often taken seriously, perhaps because it is perceived as temporary, seasonal depression can be debilitating. In some cases, it may even lead to chronic depression that does not go away with the onset of summer.

In a recent report from the Canadian Journal of Diagnosis (Canadian Consensus Guidelines for the Treatment of SAD; source: PsychDirect.com; 2006), studies show women are four times as likely as men to experience SAD. The study also showed that most people experience SAD during winter months over a 60-90 day period, and for at least two years in a row.

Sunlight is not just an aspect of the world we live in, or food for plants and trees. Sunlight is an essential ingredient in human health. Sunlight fuels serotonin production, a neurotransmitter that helps to regulate your mood. Inadequate serotonin production is a primary cause of chronic depression, and many of the anti-depressant drugs on the market are designed to increase serotonin levels.

Adequate exposure to sunlight is also required in order to produce Vitamin D. Vitamin D deficiency is linked to many diseases and disorders, including osteoporosis, diabetes, and even cancer. The single best source of Vitamin D is sunlight; more than milk and other foods rich in Vitamin D. Studies show that Vitamin D supplements can help reduce symptoms of SAD.

While lack of sunlight is the main cause, other related factors contribute to seasonal depression. For example, with limited daylight and cold temperatures, people exercise less. Studies show that exercise releases endorphins, which give you a sense of well being. More importantly, regular exercise increases your serotonin and dopamine levels, which helps you to feel a sense of happiness and calm throughout the day.

The treatment of choice for SAD is light box therapy, specifically a fluorescent light box. With daily usage, patients usually feel noticeably better within one week. There are many different types of light boxes, and there are very specific guidelines to follow; position of the light box and duration of exposure are among the variables.

It's always best to seek medical guidance rather than self-diagnosing and purchasing your light box online. People who suffer from depression or other mood disorders in other months of the year may need medication to treat SAD, or a combination of medication and light therapy. Light therapy boxes can be expensive, but some insurance companies will cover most or all of the expense if prescribed by a physician.

Of course, one of the best ways to treat a mild case of the "winter blues" is to get out in the sun. Make a practice of taking a thirty-minute walk in the sun each day, especially during mid-day when the sun is at its brightest. You will get the benefits of sunlight exposure, fresh air, and exercise, at the same time. Go for a walk during your lunch break, or take the dog for a longer walk after work. Whatever you choose to do outside, just get out. It can make a world of difference in your mood.