The Impact of Daylight on Sleep

The key points

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

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

• The interaction between the hormones serotonin and melatonin determines how sleepy or alert we feel. Their levels are directly influenced by light and darkness.

How Daylight Affects the Eyes, Brain, Hormones & Nervous System

For a long time, the impact of light on humans was underestimated. Today, we know that without sufficient daylight, our internal clock falls out of sync, leading to noticeable consequences for sleep, health, and overall well-being.

Light regulates our circadian rhythm, boosts mood, increases alertness, and can even improve productivity. But daylight is far more than just brightness or darkness – it acts as a powerful biological signal that directly influences the impact of daylight on sleep.

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

The retina of our eyes contains different types of photoreceptors. While rods and cones enable visual perception, specialized retinal ganglion cells detect blue light and determine our sense of time.

These cells extract information about the time of day from daylight and transmit it directly to the body’s internal biological clock in the brain, making them a crucial link between light exposure and sleep regulation.

How the internal clock of our nerve cells & organs works

Every cell in our body contains tiny clocks. To keep them synchronized, a central clock in the brain coordinates all bodily processes according to the day–night cycle.

This ensures that body temperature, alertness, and hormone release follow a natural rhythm. Without daylight as a timing signal, this finely tuned system cannot function properly, which negatively affects sleep quality.

The interaction between serotonin & melatonin

Two hormones are essential for the sleep–wake cycle: serotonin and melatonin. Daylight increases serotonin production, which supports mood and daytime alertness.

Melatonin, the sleep hormone, is mainly released in darkness. Since melatonin is produced from serotonin, insufficient daylight during the day can lead to reduced melatonin production at night – resulting in sleep problems, daytime fatigue, and a clearly diminished impact of daylight on sleep.

How to Align Your Biorhythm with the Natural Day–Night Cycle

Do you remember those childhood days when you spent hours playing outside? You may also remember that pleasant physical tiredness in the evening that made falling asleep effortless. This effect is not only caused by movement or fresh air, but primarily by daylight exposure.

Studies show that the more natural daylight we are exposed to, the easier it is to fall asleep at night and the better overall sleep quality becomes. To support this process, conscious management of light and darkness throughout the day is key – and has a direct impact of daylight on sleep.

Below are practical tips to help align your biological clock with your environment:

• Morning: Get outside as early as possible, ideally right after waking up. Just 30 minutes of daylight are enough to stimulate serotonin production and signal your body to become active.

• Daytime: Spend as much time as possible in bright spaces with natural light. Place your desk near a window or step outside during your lunch break. Even on cloudy days, outdoor light is significantly stronger than artificial lighting.

• Evening: Create a low-light environment and avoid screens at least 30–60 minutes before bedtime. Blue light from phones, laptops, and TVs suppresses melatonin production and delays sleep onset.

• Night: Ensure complete darkness in the bedroom. Blackout curtains or a sleep mask support melatonin release and help you sleep through the night.

Light Therapy for Sleep Issues: Daylight Lamps vs. Natural Light

For persistent sleep problems, shift work, jet lag, or during the dark winter months, light therapy using special daylight lamps can be helpful. These lamps have an intensity of 10,000 lux, simulating natural daylight.

During this therapy, position yourself in front of the lamp for about 30 minutes, ideally in the morning between 6 and 10 a.m. This helps to align your internal clock optimally with the time of day.

However, real daylight remains the best solution. Being outdoors provides not only light but also fresh air and physical activity – factors that further enhance sleep quality. Daylight lamps are a useful tool but cannot replace daily time spent outside.

Conclusion

Daylight is the strongest regulator of our internal clock and has a direct impact on sleep. Special ganglion cells in the retina transmit information about the time of day to the brain, where millions of molecular clocks in our cells are synchronised.

The interplay of serotonin and melatonin then determines whether we feel awake or sleepy – and this balance can only be maintained with sufficient daylight. With at least 30 minutes of natural light per day and conscious darkness in the evening, you create the ideal conditions for restorative sleep.