When the days are short, the sun barely shines through urban canyons, and office lighting becomes our only source of light, researchers speak of ‘living in biological darkness’. In fact, participants in a Berlin study received little more than 80 lux for around 70 per cent of their active time on winter days. The values were only just over 500 lux for 36 minutes a day. By way of comparison, in bright sunlight outdoors, 50,000 lux or more would be normal.
But what is so special about light conditions in the city? The many buildings, narrow streets and the fact that we often spend time indoors ensure that we get even less natural daylight here than we usually do at this time of year. Even when the light outside is inviting, only a fraction of it reaches our eyes. This can easily increase in urban environments. You leave your apartment in the dark in the morning, take the underground to the office, sit far away from the window during the day (or are completely shielded from it), and possibly go home in similarly dismal darkness in the evening – day after day.
What does this mean for our body? Along with sleep and food, light is one of our most important zeitgebers, controlling the day-night rhythm, influencing hormones and thus our mood. This is particularly evident in the study of REM sleep, a sleep phase that is highly dependent on the internal clock. Those who experienced extremely little light at midday showed a noticeable shift in REM phases at night: shortened REM latency, more REM at the beginning of sleep – a pattern that was once considered a possible ‘depression biomarker’.
We don’t all fall prey to winter depression, but the results show how sensitively our brain reacts to slight differences in light. A lack of motivation, tiredness and lethargy, as well as the need for more sleep, are just some of the possible reactions.
A short walk or at least a place by a window, even in an urban environment, can help to improve your ‘light balance’. Artificial light should be of high quality and as bright as possible – daylight lamps, with around 5,000–6,500 Kelvin, similar to natural daylight, can help. Those who are particularly sensitive or feel listless often benefit from adding half an hour of light therapy with such a lamp to their usual shower in the morning. 2,500 lux is helpful for this, but professional lamps provide at least 10,000 lux.
Nowozin C et al. Living in Biological Darkness II: Impact of Winter Habitual Daytime Light on Night-Time Sleep. Eur J Neurosci. 2025;61(2):e16647.