Life in a 24-Hour Rhythm

For billions of years, life on Earth has followed a stable rhythm: a day lasts 24 hours, with light during the day and darkness at night. Almost all organisms—from bacteria to humans—have evolved in adaptation to this cycle. Periods of activity and rest are governed by an internal clock.

In the 1970s, researchers first identified genes that regulate this rhythm: the so-called clock genes. These “timekeeping genes” coordinate our sleep–wake cycles, metabolism, and many other bodily functions. In 2017, three U.S. scientists were awarded the Nobel Prize in Medicine for uncovering this mechanism.

The central control center of this internal clock is the suprachiasmatic nucleus (SCN)—a tiny region in the hypothalamus, located deep within the brain. It can be thought of as the conductor of an orchestra, setting the tempo that all other bodily rhythms follow. This is why the SCN is considered the body’s master clock. When this region is damaged, regular biological rhythms can be completely lost.

But how does this internal clock, hidden deep inside the brain, know whether it is day or night outside? To stay aligned with the external world, the SCN depends on an external time cue—and that cue is light.

Light, Photoreceptors, and Blue Light

Light information reaches the brain via the retina. There, we find the classical photoreceptors—sensory cells that respond to light: rods and cones. Rods enable vision at dusk and at night, when light levels are low. Cones operate in daylight and allow for color vision and visual sharpness. For a long time, scientists believed that only these two types of photoreceptors provided information relevant to our daily rhythm.

It was not until the early 2000s that a third type of receptor was discovered: intrinsically photosensitive retinal ganglion cells, or ipRGCs. “Intrinsically photosensitive” means that these cells are light-sensitive by themselves. They contain melanopsin, a pigment that responds particularly strongly to short-wavelength blue light.

What makes these ipRGCs special is that their function is entirely different from that of rods and cones. They are not involved in vision at all. Instead, they act as pure messengers, informing the SCN whether it is light or dark outside. In this sense, ipRGCs form a window to the brain’s otherwise completely light-shielded timekeeper. Our internal clock, which runs on an almost exact 24-hour cycle, uses this light input in a complex feedback loop to fine-tune itself—constantly recalibrating to the external environment.

Blue light is therefore the decisive external time cue for our internal clock and our sleep–wake rhythm.

Blue Light, Screen Use, and Marketing Promises

In recent years, blue-light-filter glasses have been aggressively marketed. They are said to reduce “digital stress,” protect the retina, and improve sleep. However, there is no solid scientific evidence to support these claims—simply put, there are no convincing studies demonstrating such effects.

To begin with, “digital stress” is not a clearly defined medical term. Fatigue, headaches, or concentration problems after prolonged screen use are mainly caused by other factors: sustained close-up focusing that tires the eye muscles, reduced blinking that leads to dry eyes, and a lack of breaks. Not by blue light.

The concern that screen use might damage the retina is also not supported by scientific evidence. Yes, extremely high intensities of blue light can cause photochemical damage—that is, damage caused by chemical reactions triggered by light. But the light emitted by screens is far weaker than the levels we are exposed to in normal daylight. There is no convincing evidence that typical screen use causes retinal damage.

What remains unclear, however, is whether and how blue-light-filter glasses might interfere with our natural daylight rhythm. After all, as we have learned, blue light is the most important time cue for our internal clock.

Blue Light and Sleep

This is where things become interesting. Blue light suppresses the release of the sleep hormone melatonin—and during the day, this is biologically useful, as it supports alertness and performance. As darkness sets in, melatonin levels rise and we become sleepy. We need blue light during the day.

It is only in the evening that blue light can become problematic. Bright screen lighting may delay the natural transition to darkness and make it harder to fall asleep. But even here, the solution is not as simple as it is often portrayed. There is no solid evidence that a mere “filter” has a meaningful positive effect. If anything, a true blue-light blocker—one that blocks blue light completely, rather than merely reducing it—might be relevant during evening screen use.

The arguments put forward by blue-light-filter manufacturers therefore rest on shaky ground. They promise protection against problems that are either not caused by blue light at all (eye strain, retinal damage) or for which simple filters may not be sufficiently effective (sleep problems in the evening).

A more sensible approach would be to seek plenty of daylight exposure during the day, take regular breaks when working on screens, and reduce digital device use in the evening. The “Night Shift” mode that many devices activate automatically, by the way, has only a modest effect.

The best strategy remains good sleep hygiene: regular bedtimes, a dark bedroom, avoiding stimulating content before sleep—and yes, reducing screen time in the evening. Not because of blue light alone, but because digital devices keep our minds alert and active.