Persons with mood illnesses are more sensitive to light than others (see the current article of the month below): Too much light causes manic symptoms, too little light causes depressive symptoms. In the spring and summer there is too much light, in the fall and winter there is not enough light. You thus need to modulate your light exposure to avoid causing mania in the spring and summer and depression in the fall and winter. Mood episodes go together, so if you want to prevent winter depression, you should also try to prevent summer mania.
In winter, there is not enough light. Therefore you will sleep in the morning longer than you should, and that will throw off your circadian rhythms. You therefore need to increase your light exposure.
Plan on spending $200-300. There is no need to spend more. The dose is 10,000 lux (most lightboxes are at that dose). There are many companies - the internet engines will help you. Use indirect light; do not look into the light box directly (treat it like the sun). Have it on the table while you read or eat in the morning. Dose is 25- 30 minutes to one hour as long as depression persists. Use the lightbox daily initially, and then reduce use to every other day for prevention. Use the lightbox in the morning (before 10 AM): remember you're trying to replace the morning sunlight which declines in winter.
In summer, there is too much light. Therefore you will sleep in the morning less than you should, and that will throw off your circadian rhythms. You therefore need to decrease your light exposure:
There are simple guidelines that all patients can follow: Lift the shades in the winter, and pull them down in the spring and summer. Use an eyemask (or get room darkening shades) in spring/summer. If you're still depressed in winter despite raising the shades, use a light box in the morning as explained above.