The southward movement of the sun also signals birds to gorge food in preparation for their long migration. Photobiology studies the interaction of light with living things and how we can protect ourselves from any negative effects. Its impact on human society is profound. Social and economic factors these days tend to disrupt the natural circadian rhythms our bodies require over the course of 24 hours. Light is a primary factor that influences psychological and physiological changes affecting our sleep, alertness and emotions.
Seasonal changes affect people profoundly, too.
During September and October, as darkness begins to infringe on our normal activities, we notice the sun's withdrawal most acutely. This may alter or curtail our outdoor activities long before the weather does. The psychological impact on human society as a whole is profound. Throughout the centuries, poets have described a sense of sadness, loss and lethargy which can accompany the shortening days of fall and winter. Many cultures and religions have winter festivals associated with candles or fire.