From CpapWiki
Light Therapy
The short and sweet - during winter months - especially in northern areas - the lack of light leads to depression.
More light means less seasonal affective disorder.
Light boxes are made specifically for this use - very bright and safe for the eyes. Don't use incandescent bulbs, and be ware of home made lights unless you read up on it first.
Particularly effective if used in early morning. Most are made to use at least 30 minutes per day, but using them longer than that at your desk at work can work even better, and lets you sit farther away which might be desirable.
See, Winter Blues by Norman Rosenthal, former director of the light therapy department at NIH, for more info.