Light is needed for vision, but light entering our eyes also has other “non-image forming” effects. It plays an important role in how we feel and how well we can function mentally. Reduced sunlight in winter may cause winter-onset SAD (seasonal affective disorder) or seasonal depression.
Some of the non-image forming effects of light exposure depend on the colour of light in combination with timing of exposure. Some colours and intensities that are beneficial at one time of day may be detrimental at other times of day. For examples, blue light, which is emitted by the sun during the day, is good at keeping us awake and energetic so can be used to help combat seasonal depression.