Light and darkness are the strongest influences on our internal clock; they synchronise us with the earth's day-night cycle. Light lands on special receptors in our retina and switches us to "activity" mode, while darkness causes us to release melatonin from our pineal gland and switches us to "sleep" mode
However, it is now the case that 80% of people in Germany work in closed rooms. Even if workers themselves perceive the rooms to be brightly lit, they mostly only feature illumination levels of 600 lux, where the human body requires at least 2500 lux in the mornings to properly wake up. In spring and summer, this level can attained with daylight coming in from the outside, but there is an insufficiency of light in autumn and winter. In the dark periods of the year, low light means that poor mood, fatigue, sleep disorders and depressive states are seen much more frequently.
Light deficiency has a negative effect on mood, makes it more difficult to stay awake and disrupts the sleep-wake rhythm. The impact of light deficiency includes a lack of serotonin – a messenger substance that lifts mood – in the brain, and an excess of the darkness hormone melatonin in the morning.
Light therapy with a daylight lamp can help compensate for light deficiency. Melatonin production can be stopped if the daylight lamp is used as soon as you wake up in the morning, so as early as possible; as a result, there is a positive uplift in your mood.
Source: Schlafen für Aufgeweckte, Dr. med. Michael Feld, 3. Auflage 2015