Have you ever noticed how different the world looks at sunrise compared to midday or sunset?
In the early morning, daylight has a warm, golden glow. Around noon, it appears bright and crisp. As evening approaches, the light becomes soft, orange and eventually fades into darkness.

These changes are more than beautiful scenery. They are powerful biological signals that have shaped human physiology for millions of years throughout our evolution. Our bodies have evolved under the predictable daily rhythm of the sun, and our biological clock still relies on these changing light conditions to regulate sleep, alertness, mood, hormone production and overall health.

Understanding how the light spectrum changes throughout the day helps explain why timing matters just as much as the amount of light we receive.

WHAT IS THE LIGHT SPECTRUM?
Visible light consists of many different wavelengths, each perceived as a different color.
The spectrum ranges from shorter wavelengths, which we see as blue and violet, to longer wavelengths, which appear yellow, orange and red. Sunlight contains all of these colors, but their proportions change throughout the day.

Our eyes use this information from the spectrum not only for vision but also to inform the brain about the time of day. Specialised light-sensitive cells in the retina, called intrinsically photosensitive retinal ganglion cells (ipRGCs), or as we call them: spheres, communicate directly with the brain’s master biological clock, helping regulate circadian rhythms.

SUNRISE: A GENTLE WAKE-UP CALL
Just before and after sunrise, sunlight travels through a thick layer of the Earth’s atmosphere. During this journey, much of the shorter blue wavelengths are scattered, allowing more longer wavelengths, reds, oranges and warm yellows, to dominate what we see. Sunrise is an important signal for the biological clock. Even relatively low levels of natural daylight are far brighter than typical indoor lighting and begin preparing the body for the day ahead. As the sun rises, the intensity increases rapidly and more blue wavelengths become available, helping promote alertness.

MORNING: BUILDING BIOLOGICAL MOMENTUM
As the sun climbs higher, daylight becomes brighter and richer in shorter wavelengths.
This combination of increasing light intensity and greater blue content tells the brain that daytime has fully arrived. For this reason, spending time outdoors in the morning is one of the most effective ways to strengthen your biological clock.

MIDDAY: NATURE’S STRONGEST LIGHT
Around midday, sunlight reaches its highest intensity.
The sun is high in the sky, meaning its light passes through less atmosphere. As a result, the full spectrum reaches the Earth’s surface with relatively little filtering and is rich in blue light. Even on cloudy days, outdoor light often provides many times more illumination than a well-lit office.
The brightness of midday light plays a crucial role in reinforcing the difference between biological day and biological night.

AFTERNOON: A GRADUAL TRANSITION
As afternoon progresses, the sun slowly moves lower.
The spectrum begins shifting again as sunlight passes through more atmosphere. The proportion of blue light gradually decreases while warmer colours become increasingly prominent. At the same time, overall light intensity starts to decline.
This gradual change acts as a natural transition, allowing the body to begin preparing for the evening without an abrupt shift.

SUNSET: PREPARING FOR NIGHT
During sunset, the atmosphere filters out much of the shorter blue wavelengths. The remaining sunlight appears rich in orange and red tones, creating the warm colors often associated with the “golden hour.”
This warmer spectrum, combined with rapidly decreasing light intensity, signals to the brain that daytime is ending. As darkness approaches, this stimulates the body to prepare for sleep.
The transition from bright days to dim evenings is one of the most consistent environmental signals our circadian system has evolved to recognize.

NIGHT: DARKNESS IS PART OF THE SPECTRUM TOOALTHOUGH WE OFTEN THINK ABOUT DAYLIGHT, DARKNESS IS EQUALLY IMPORTANT
For thousands of generations, humans experienced nights illuminated only by moonlight, starlight and fire, all relatively dim light sources with very limited biological impact compared to modern lighting.
Today’s homes often remain brightly illuminated well into the evening using LED lighting that contains substantial amounts of blue light. This creates an environment that no longer resembles the natural progression from day to night.
Instead of experiencing a gradual reduction in light intensity, many people effectively extend their biological daytime by several hours.

WHAT MODERN LIGHTING GETS WRONG?
Modern indoor lighting is remarkably consistent.
Many offices, homes and public spaces use the same lighting throughout the entire day, often with similar brightness and colour temperature from morning until bedtime. This is very different from the dynamic lighting environment humans evolved under.

This weak distinction between day and night can reduce circadian contrast and make it more difficult for the body to know when to be alert and when to prepare for sleep.
The body’s response depends on the entire daily light history, not just a single moment.

SIX STEPS TO GOOD LIGHT
While we cannot always work outside, we can design our environments to better reflect the natural rhythm of daylight.
– Give your eyes a few minutes of bright daylight in the morning. This signals to your body that the day has begun.
– Spend at least two hours outdoors during the day, especially in the morning.
– Sit as close to a window as possible during the day.
– Not enough daylight? Use electric lighting that provides at least 500 lux at eye level.
– Keep light levels below 10 lux three hours before bedtime. Dim the lights, close the curtains, avoid bright screens, and switch your phone to night mode.
– Keep light levels below 1 lux at night, even when getting out of bed, and avoid screens whenever possible.

These simple changes help recreate the natural contrast between biological day and biological night.

Author: Good Light Group
Source: https://www.goodlightgroup.org/post/the-light-spectrum-throughout-the-day-why-nature-changes-colour