The Impact of Daylight on Sleep

The key points

• Special cells in the retina of our eyes detect daylight and send information about the time of day directly to the brain, where our internal clock is located.

• The biological clock regulates millions of molecular clocks in every cell of the body. These influence body temperature, hormone release and sleep pressure.

• The interaction between the hormones serotonin and melatonin is linked to our sleep-wake rhythm. Hormone levels change depending on light and darkness.

How daylight affects the eyes, brain, hormones & nerve cells

For a long time, the influence of light on humans was underestimated. Today, we know that daylight is an important factor for our internal clock and is linked to our sleep-wake rhythm, well-being and various processes in the body.

Light can influence our circadian rhythm, attention and mood. But daylight is about much more than simply light or darkness – while we move through the world with our eyes open, processes take place in the background that have nothing to do with visual perception.

Ganglion cells send information about the time of day to the brain

There are different types of photoreceptors in the retina. While the classic rods and cones allow us to perceive colours, details and movement, there is another, lesser-known type of cell in the retina: retinal ganglion cells.

These specialised cells are particularly responsive to short-wavelength components of daylight, often referred to as blue light. Their role is to extract important information about the time of day from our surroundings and send it directly to the biological clock in the brain.

How the internal clock of our nerve cells & organs works

Every cell in our body contains molecular clocks. To keep these millions of molecular clocks synchronised, we have a central clock in the brain – more precisely, in the hypothalamus. Like the conductor of an orchestra, it communicates information about the time of day to the body and influences processes in line with the day-night rhythm.

As a result, our body temperature rises in the morning and falls in the evening. At the same time, hormones are released at appropriate times, helping us feel alert and awake or tired and sleepy. This interaction between our internal circadian rhythm and external time cues relies on the body receiving information about the time of day – including through daylight.

The interaction between serotonin & melatonin

Two hormones are particularly relevant to the sleep-wake rhythm: serotonin and melatonin. Serotonin is involved in various processes in the body. Melatonin, on the other hand, is closely linked to the sleep-wake rhythm and is primarily released in darkness.

The interesting part is that melatonin is produced in the body partly from serotonin. Light conditions can influence the natural daily rhythm of melatonin release. A regular alternation between light and darkness can therefore support a stable sleep-wake rhythm.

How to adjust your circadian rhythm to the time of day

Can you remember those days in childhood when you spent the entire day outdoors? Then you probably know that pleasant feeling of physical tiredness that comes in the evening. This is not only related to physical activity or fresh air, but also to daylight.

Studies show that exposure to natural light during the day is linked to the sleep-wake rhythm. To support a regular rhythm, there are several things you can do. The key is being conscious of light and darkness throughout the day.

Here are our tips for structuring your day in a way that helps bring your biological clock and daily routine into balance:

• In the morning: Go outside as early as possible – ideally shortly after getting up. Morning daylight can help your body adjust to the day ahead.

• During the day: Try to spend as much time as possible in bright rooms with natural light. Place your desk near a window or go outside during your lunch break. Even on cloudy days, outdoor light is generally much brighter than artificial lighting.

• In the evening: Use dimmed lighting and reduce screen time before going to bed. The blue light from your phone, laptop and TV can influence melatonin release.

• At night: Keep your bedroom as dark as possible. Blackout curtains or a sleep mask can help reduce unwanted light during the night.

Light therapy for sleep problems: daylight lamps vs. natural light

For persistent sleep problems, shift work, jet lag or during the darker winter months, light therapy with specialised daylight lamps may be an option. These lamps often have an illuminance of up to 10,000 lux and are designed to simulate bright daylight.

With this type of light therapy, you typically sit in front of the lamp for around 30 minutes; depending on the application, it is often used in the morning between 6 and 10 am. Follow the manufacturer's instructions for the specific application.

Natural daylight is easy to incorporate into your daily routine. When you spend time outdoors, you get not only light but can also combine fresh air and physical activity with your daily routine. Daylight lamps can therefore be an additional option, but they do not replace spending time outdoors.

Conclusion

Daylight is an important regulator of our internal clock and is directly linked to our sleep-wake rhythm. Special ganglion cells in the retina send information about the time of day to the brain, where millions of molecular clocks in our cells are synchronised.

The interaction between serotonin and melatonin is linked to our state of wakefulness and sleep. Being mindful of natural light during the day and darkness in the evening can help support a regular sleep-wake rhythm.