Many physical and mental processes are controlled by an inner clock. During the winter months, the lack of light causes a higher production of the sleep hormone melatonin and we sleep more, but often have less energy during the day, as the production is decreased by bright daylight falling into our eyes. A lack of light has also been shown to lead to depression (winter depression, SAD). However, the brightness indoors is often lower than outdoors in the sunshine, even in summer. Sunlight or therapy light, when used correctly, brings the inner clock back into sync and thus ensures that the neurotransmitter serotonin and other messenger substances will again be present in higher concentrations so that they can have a regulating effect in the brain. The brain messenger serotonin, along with others, ensures the transmission of information from brain cell to brain cell and is known as the "feel-good hormone". Serotonin deficiency is the cause of many depressions. In addition, disorders of other messenger systems (noradrenalin, dopamine, glutamate and others) can also play a role.  When the transmission of information functions well again, one can think better, feel better, sleep better, enjoy life more intensively again, feel happier and more balanced again.  Inhibition of thinking, inhibition of emotions, concentration, sleep and behavioural disorders, such as anxiety, decrease significantly or disappear completely. Light therapy is now also successfully used for sleep disorders, jet lag (time zone changes), old-age depression and disturbances of the inner clock due to shift work. Even healthy people can demonstrably increase their overall well-being and performance through light.