A key debate in our modern era is whether the blue wavelengths emitted from our lights and devices disrupt our sleep.
For the first 10,000 generations of human existence, we never had to worry about exposure to blue light in the evenings. Wood fires and candles contained virtually no blue, and nights were dark. But in the last ten years, we have entered the era of ubiquitous blue-rich LEDs in our computer and mobile device screens, and in the lights in our homes and workplaces.
Some sleep scientists claim that removing evening blue light is unnecessary because it doesn’t affect sleep onset. Other experts tell you to protect your health by avoiding blue-containing light after sunset. Who is correct? It turns out, like many debates, it is all a matter of definition of what you mean by “blue”, and what you mean by “disrupted sleep”.
This week, guest writer Dr Martin Moore-Ede M.D., Ph.D., author of Light Doctor, explores the different perspectives and research-led insights on the effects of blue light on our sleep, and what we can do to ensure a healthy sleep routine.
All blue light is not the same
“Blue” is a remarkably loose term. The colour range that people call “blue” varies between cultures and scientific disciplines. The ancient Greeks did not have a word for blue in their language. Japanese has one word, “ao”, for blue and green, which is the reason that Japanese traffic lights use a blueish-green color. In contrast, the Russian language has two words for blue.
Because the word “blue” is so vague, defining the wavelengths of blue light matters. Some scientists define blue as all light wavelengths between 400 and 500 nm; others define a narrower range. The term “blue” can include a broad range of colours from violet (400 - 425 nm), indigo (425 - 450nm), royal blue (450 - 465 nm), sky blue (465 – 490 nm), and aqua (490 - 500 nm). But even these color wavelength ranges are inconsistently used between authors and scientists.
It is much better to define blue wavelength ranges in terms of their physiological effect. With regard to sleep, the most important definition of blue is the wavelengths that reset the circadian clock. Circadian blue has peak effectiveness at 480 nm, with a full-width half maximum (FWHM) range of 440 – 495 nm under normal, fully light-adapted conditions. It triggers a photopigment called melanopsin with a peak sensitivity at 480 nm in the intrinsically photosensitive retinal ganglion cells (ipRGCs) in the eye that communicate with the master circadian pacemaker, the suprachiasmatic nucleus (SCN) and can reset the circadian clock.
Circadian blue is highly potent, even at low doses. Pure (monochromatic) circadian blue light at its peak effective wavelength of 480 nm is 20 times more effective at resetting circadian clocks than regular polychromatic white light.
Be aware that this information gets garbled by ChatGPT and DeepSeek, and in published models of circadian light effects. This is because they fail to recognise that most of the early studies were conducted in people studied in the dark with fully dark-adapted eyes, where there is a much wider circadian sensitivity range that includes violet and green wavelengths.
The timing of blue light matters
Circadian blue light (440-495 nm) can be either good or bad for sleep depending on the time of day. Daytime circadian blue light, especially in the mornings, helps robustly synchronize circadian clocks, increasing their amplitude and stability, and boosting melatonin and healthy consolidated sleep at night.
However, circadian blue wavelengths in the nocturnal hours from sunset to sunrise can disrupt circadian clocks, suppress melatonin, and disturb sleep.
Blue light dosage is critical
Natural daylight, which is rich in circadian blue wavelengths, is so effective because it is 100 – 1000 times brighter than typical indoor artificial electric light levels. That is why getting outside every morning is so critical for healthy nocturnal sleep.
If you must stay indoors during the day, then you need to make sure at least 20 µW/cm2 of 440-495 nm circadian blue light is falling on your eyes to maintain good quality sleep and circadian health. This cannot be easily achieved at comfortable brightness levels using conventional LED lights as they have a dip in spectral power between 460 and 490 nm. However, spectrally optimized circadian day LED lights with a spectral peak at ⁓480 nm can provide sufficient circadian blue.
After sunset, it is important to reduce indoor circadian blue dosage to less than 2 µW/cm2 of 440-495 nm circadian blue at eye level, and preferably to less than 0.4 µW/cm2. This can be achieved by using light bulbs and fixtures that contain less than 2% 440-495 nm circadian blue content, such as Soraa’s Zero Blue light bulbs or Korrus’s Circadian Blue fixtures, or by using orange-red light with a CCT of 1900 K or below.
Another solution is to use blue-blocking glasses in the evening, especially if you cannot change the lights and screens you use. Though beware - some of the biggest-selling “blue blocker” eyewear filter out blue light mostly below 450 nm. The peak circadian blue effect is at 480 nm, which these clear-looking glasses barely touch. The giveaway for placebo glasses is that they are only lightly tinted to make them look attractive. In contrast, glasses that remove the most circadian active blue wavelengths (between 440-495nm), such as BlueSafe24, are distinctly yellow-orange in colour.
The definition of disrupted sleep
Some sleep scientists have recently suggested that evening blue light is not an issue because it has minimal effects on the time you fall asleep. And it is true that in studies of people living their regular lives during the day and sleeping at night, exposure to blue light in the evening makes less than ten minutes difference in the time they fall asleep.
However, sleep onset is an insensitive measure of circadian health and sleep quality. There are at least three other significant factors that determine the time you fall asleep.
The first is homeostatic sleep drive - the pressure to sleep that is determined by the number of consecutive hours you have been awake. The longer you stay continuously awake, the greater the pressure to sleep.
The second is your level of arousal. Chemical stimulants like caffeine can delay the time that you are able to fall asleep. So can other types of arousal, such as an argument with your partner or spouse, watching a horror movie, or receiving a disturbing phone call or text. Conversely, other substances can make you sleepy and advance the time of sleep onset.
The third is our conscious ability to fight off sleepiness for a while if we have something important to do. That can be risky if you are driving a car or in some other hazardous situation because sleep pressure can override your conscious ability to stay awake.
Together these factors mask the impact of blue light on sleep onset. However, there any many other more sensitive measures of the impact of blue light on sleep, and these depend on the time of day.
Daytime effects of circadian blue light on nocturnal sleep
People exposed to less than 20 µW/cm2 of 440 - 495 nm circadian blue light at eye level during the daytime hours, when compared to people with greater daytime blue light exposure, show:
· Disrupted sleep and reduced daytime vitality
· Reduced sleep length and decreased daytime activity
· Impaired sleep and lowered daytime mood, alertness and performance
· Disrupted circadian sleep rhythm, impaired cognitive performance, and mood
· Disrupted circadian rhythms, decreased nocturnal melatonin levels, and increased daytime sleepiness
Evening effects of circadian blue light on nocturnal sleep
When people are exposed to blue-rich light of greater than 2 µW/cm2 of 440 - 495 nm circadian blue light at eye level in the evening from lamps, light fixtures, computer and TV screens, and mobile devices, then significant disruption of sleep and health results.
These include:
· Suppression of REM (dreaming) sleep
· Suppression of melatonin
· Delay and disruption of circadian clocks
· Heightened sympathetic nervous system stress response during sleep
· Impaired alertness and cognitive performance (e.g. math tests) the next day
· Reduced size of the dentate gyrus, a part of the brain that plays a critical role in memory formation and retrieval
· Impaired glucose metabolism and increased insulin resistance
· Increased risk of a wide range of metabolic, carcinogenic, and psychiatric pathologies
The danger of the harm you cannot see
The problem is that you cannot tell with the naked eye how much blue that a white, or yellowish/white, light is emitting. However, it is important to know this because the causal pathway of the wide range of disorders caused by blue-rich light at night involves melatonin suppression and circadian disruption, where the rhythms in various body functions are out of sync with each other.
This is a key contributor to the modern epidemic of obesity, diabetes, heart disease, psychiatric depressive disorders, and endocrine-sensitive cancers, such as breast and prostate. These are exacerbated by blue-rich light at night, and too little blue-rich light during the day because of spending too much time indoors.
We also tend to use the timing of our sleep - the time we fall asleep and when we awake - as a yardstick of our health, because it is something that we are so well aware of. However, sleep onset is a relatively insensitive measure of circadian health, and blue light impacts much more sensitive and important measures of our health that are not so readily apparent.
It is my hope that these insights, and much more that I go into detail about in my book Light Doctor, empower you to make informed decisions about any adjustments you feel necessary to your sleep routine, to ensure optimal health and wellbeing.
Dr. Martin Moore-Ede is a leading world expert on circadian clocks and the health problems caused by electric light at night. As a professor at Harvard Medical School (1975 – 1998), he led the team that located the suprachiasmatic nucleus, the biological clock in the human brain that controls the timing of sleep and wake, and showed how it is synchronized by light. Since 2010, he has been the Director of the Circadian Lighting Research Center, which identified the key blue signal that synchronizes circadian clocks and developed patented LED lights, which provide health-optimized light across day and night based on comprehensive medical research. He has published over 180 scientific articles and authored ten books, including his latest one, THE LIGHT DOCTOR: Using Light to Boost Health, Improve Sleep and Live Longer.