Light during the day and darkness during the night is supporting a good night of sleep and a day full of energy. Most people recognize they feel much more energetic during a bright, sunny, day and often enjoy a sound sleep afterwards. Dark, gloomy days, especially in winter may induce feelings of lethargy, low spirit and sleepiness.
The reason why some people experience mild changes in energy levels, sleep need, and mood in winter, and others do not, is not really known. Whether it actually is the lack of light that is causing the symptoms is also unclear. What is known is that light exposure through the eyes, either by the sun or by an artificial light source, is able to counteract reduced feelings of well being.
In people experiencing seasonal changes, bright light during the short winter days, and in particular during the morning hours, may fight the low energy levels and improve mood. Already 30 minutes of light is capable of increasing subjective energy levels.
If one recognizes the annual pattern in feelings of fatigue, increased sleep need and mild mood disturbances in winter, it seems optimal to start exposure to 20-30 minutes of bright light in the morning around breakfast time, as soon as the first signs in autumn appear.