Humankind evolved under the outdoor day-night cycle, and it is essential to maintaining normal, healthy physiology. Exposure to sunrise is key to synchronizing our internal body cycles to the external world (so we sleep when it's dark and are alert during the day).
Modern industrial society began to tamper with this system, though. The loss of daylight - in dark apartments, in the workplace, on shift work, etc. - creates a state of daylight deprivation, making us vulnerable to depressed mood, sleep problems, fatigue, ability to concentrate, and more. Nature, too, posed a new problem when humans began to migrate north, where winter nights are long and dark. Although researchers don’t have firm numbers, lots of people are light deprived. Symptoms of light deprivation include:
1. Feeling depressed or moody during the fall and winter
2. Fatigue, lack of energy, and difficulty getting up in the morning
3. Problems getting things done (lack of motivation)
4. Reduced social contact (often reduced sexual interest)
5. Cravings for carbohydrate and weight gain

Surveys suggest that as many as 1 in 20 people experience full-blown winter depression in the northern hemisphere, and three times as many experience winter doldrums. Far larger numbers experience nonseasonal depression at some point in their lives, often for years at a time. With the recent finding that light therapy can also work for nonseasonal depressions – light itself or in combination with drugs, the size of the responsive population multiplies. We’re talking many millions of folks. . . .