How the color of light impacts our internal clocks and circadian rhythms

Circadian rhythms are your body’s built-in 24-hour timer that keeps everything running smoothly, from your sleep schedule to your mood and even how you digest food. It’s like an internal clock that syncs you up with the world around you.

These rhythms are mostly guided by light and dark — sunlight tells your brain it’s time to wake up, while darkness signals that it’s time to wind down.

The boss of this system is a tiny part of your brain called the suprachiasmatic nucleus (SCN), which sits in the hypothalamus. It’s basically the control center that keeps you on track with the day-night cycle.

Your internal clock doesn’t just decide when you should sleep — it also impacts your energy levels, hormones, and how your body works throughout the day.

We know that the brightness of light surround us plays a major role in regulating your circadian rhythms, but what about the color of the light?

Dr. Christine Blume from the Centre for Chronobiology of the University of Basel has examined how these signals to the light receptors in our eyes might involve not only brightness but also color.

“There is reason to believe that the color of light could also be relevant for the internal clock,” says Dr. Blume.

Lights, colors, and circadian rhythm

Human eyes rely on specialized cells to gather light information. Rods work best in dim conditions, providing vision at low brightness levels.

Cones function under brighter conditions, aiding color perception. Light-sensitive ganglion cells, responding strongly to short wavelengths at around 490 nanometers, help inform the internal clock that it is daytime.

When exposed to light consisting solely of short wavelengths between 440 and 490 nanometers, individuals perceive it as blue. If this triggers ganglion cells, the internal clock receives signals associated with daytime conditions.

“The light-sensitive ganglion cells also receive information from the cones. This raises the question of whether the cones, and thereby the light color, also influence the internal clock,” says Dr. Blume.

“After all, the most striking changes in brightness and light color occur at sunrise and sunset, marking the beginning and end of a day.”

Previous research on mice suggested that yellowish light might affect the internal clock differently than bluish light.

The idea was that the color aspect of light, not just brightness or wavelength intensity, might shift human circadian timing.

Testing lights and colors in the lab

To test this possibility, a study exposed 16 healthy volunteers to either bluish or yellowish light for one hour late in the evening. A white light served as a control condition.

The light stimuli were designed so that differences in color-sensitive cone activation occurred without altering the stimulation of the ganglion cells.

“This method of light stimulation allows us to separate the light properties that may play a role in how light effects humans in a clean experimental way,” says Manuel Spitschan, Professor of Chronobiology and Health at the Technical University of Munich.

In a controlled sleep laboratory, researchers assessed whether the color differences influenced each person’s internal clock and sleep.

They measured how quickly each volunteer fell asleep, examined the depth of their sleep, noted how tired they felt, and recorded their reaction times, which drop as sleepiness grows.

What did the study teach us?

Study results indicated that the color difference between bluish and yellowish light had no meaningful impact on the human internal clock or on sleep patterns.

The findings ran counter to earlier mouse-based research that suggested color might matter.

“We found no evidence that the variation of light color along a blue-yellow dimension plays a relevant role for the human internal clock or sleep,” says Dr. Blume.

“Rather, our results support the findings of many other studies that the light-sensitive ganglion cells are most important for the human internal clock,” she adds.

“Our findings show that it is probably most important to take into account the effect of light on the light-sensitive ganglion cells when planning and designing lighting. The cones and therefore the color play a very subordinate role,” says Spitschan.

Why does any of this matter?

The study’s outcome focuses attention on reducing short-wavelength light if the goal is to support healthy sleep and stable circadian rhythms.

The presence of certain wavelengths matters more than shifting the color balance along a blue-yellow range.

Health advice often emphasizes reducing so-called blue light from device screens in the evening. Many people rely on night shift modes, which add a yellowish tint, believing it lessens the impact on sleep.

The research suggests that such color changes may not be as necessary as once assumed.

“Technologically, we can reduce the short-wavelength light in these displays without altering the color, but commercial mobile phones have yet to adopt this approach,” says Dr. Blume.

Circadian rhythms and human health

This work leaves questions for future studies, such as whether longer exposure times, different testing periods, or other conditions could reveal effects not seen in this setup.

In short, results here highlight the complexity of human vision, sleep regulation, and internal timing. The sensitivity of ganglion cells to short-wavelength light remains central to maintaining the body’s daily rhythms.

In everyday life, people still receive frequent advice to limit exposure to bright, blue-rich screens before bedtime. The takeaway from this study is that color adjustments may not hold the key.

The intensity and type of wavelengths that affect these specialized ganglion cells appear to be what truly matters.

Designers of lighting systems may focus their efforts on controlling the properties of short-wavelength light, rather than shifting colors along a blue-yellow axis.

For now, the findings bring attention to which aspects of light should be managed to keep the internal clock running smoothly.

The full study was published in the journal Nature Human Behavior.

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