Blind people can feel the effects of light

We need light for more than just seeing! Light also affects wakefulness and sleep, mood and concentration – and much more. Reason enough to take a closer look: What does light do to us? When do we need light and when is it harmful? And how does all this work in the body?

Cones recognize colors, rods see even in dim light: This is probably what school taught you about photoreceptors. But there are more cells in the retina that respond to light: the photosensitive ganglion cells – less famous, but just as important. They control, for example, the sleep-wake cycle. Here’s how it works: For a while, our internal clock can run on its own. To be more precise, a small collection of nerve cells in the hypothalamus, the so-called suprachiasmatic nucleus, sets the rhythm: It determines when the hormone melatonin is released, making us sleepy. But like an old fob watch, the suprachiasmatic nucleus has to be set from time to time – by light.
Bright light suppresses melatonin production and makes us feel more alert. In the evening, of course, this is usually unwelcome: Low melatonin levels not only make it difficult to fall asleep, but also reduce sleep quality. For this reason, sleep researchers recommend avoiding bright light in the hours before bedtime and keeping the bedroom dark.
But we can also take advantage of the effects of light: Bright light early in the day helps shift our circadian rhythms forward, making it easier to fall asleep at night and wake up in the morning. This can help not only night owls, but also jet-lagged travelers and shift workers. Despite many products on the market, nothing beats simply stepping outside: Even so-called daylight or full-spectrum lamps are nowhere near as effective as a cloudy December sky.

Light beyond seeing: from sleeping to sneezing

The research group Translational Sensory and Circadian Neuroscience led by Manuel Spitschan investigates what exactly the eye tells the brain – beyond visual impressions. The team also measures light conditions in the real world outside the lab, mapping when and where we are exposed to light – and to what kind of light. Because not all light is the same! The photosensitive ganglion cells, which control our daily rhythms, respond primarily to short-wavelength, or blue, light. This is why it is important to record the exact type of light exposure: in residential areas, parks and city centers; in offices and factory halls; in bedrooms as well as in public spaces. Such data can form the basis for recommendations on how to cost-effectively change lighting conditions to contribute to good physical and mental health.
By the way: Light has a number of other effects on us, some of them more obscure. Have you ever squinted into the sun and suddenly had to sneeze? Then you are one of the people who experience a photic sneeze reflex. What lies behind this phenomenon is still unclear. The Translational Sensory and Circadian Neuroscience research group is also trying to shed light on this.