What does blue light do to your circadian rhythm?

The short answer

This part simply holds. Blue light suppresses melatonin, shifts your body clock and changes when you get sleepy. It has been established cleanly in the laboratory and it is not controversial.

What usually gets left out: colour is not the only thing that counts, and usually not even the main thing. How much light there is, and above all when, weighs more. And the difference between two people looking at the same lamp is wider than the difference between two lamps.

That is why this article exists. If you want to understand why blue light glasses disappoint in the research while the mechanism looks so convincing, you first need to know how the mechanism actually works.

How does blue light affect your body?

Alongside the rods and cones you see with, your retina holds a third kind of light sensitive cell. Those cells contain melanopsin and they are not there to make images. They report what time it is ^[10].

They are most sensitive around 480 nanometres, the blue of a clear sky. That is no accident: for hundreds of thousands of years that signal reliably meant daytime. Your clock still listens to it, even when the signal now comes from a phone.

That is the dose response curve the whole industry leans on. It is real.

Which wavelength of blue light shifts your clock?

This is the core of the case for an amber or red lens: take out the short wavelength band and your clock notices the light far less.

But note what was measured. This is narrow band light in a laboratory, at a known intensity under controlled conditions. A lamp in your living room or a screen in your lap is not narrow band light, and you are not a participant with a fixed pupil.

Why is it different at home than in a lab?

In other words: some people are sensitive to evening light and others much less so, and that is a property of the person, not of the evening. Advice that is the same for everyone goes too far for some people and not far enough for others.

That gets sharper still when you look at age.

In preschoolers, melatonin stayed below half of their own baseline for at least fifty minutes after the light was already off in 62 percent of participants, and recovery did not depend on how bright the light had been ^[7]. That is a sensitivity you do not see in adults.

In older people the reverse happens. The lens of the eye yellows, so less blue light reaches the retina, and the number of melanopsin containing ganglion cells declines with the years ^[10]. In a large study of over two hundred older participants pupil responses were indeed weaker, though that turned out not to be specific to blue ^[9].

Is blue light from your screen bad for your sleep?

The best known applicable trial is the one with e-readers.

Do take in what was actually compared: a screen against a book, so light against almost no light. That is a bigger contrast than any lens with or without a blue filter can create, and it partly explains why the results for those glasses are so much more modest.

Outside the laboratory the picture is surprisingly consistent. In a field study where people went about their lives at home with sleep recording equipment, later first exposure to more than ten lux was associated with more awakenings during the night that followed ^[6]. Ten lux is not much. That is the hallway, not a sunbed.

And a systematic review of light exposure and circadian rhythm arrived at the core message that mistimed exposure disrupts the rhythm ^[3]. Not bright light as such. Bright light at the wrong moment.

In adolescents, the group the concern centres on, the conclusion does not match the headlines: evening light can suppress melatonin and delay phase, but how your light is patterned across the whole day softens or sharpens that effect ^[8]. Someone who gets enough daylight during the day is less troubled by the same screen at night.

What does this mean for blue light glasses?

Now you can see where the reasoning leaks.

Blue light suppresses melatonin: yes. A filter that removes blue should prevent that: on paper, yes. But in the randomised research on the glasses themselves melatonin was almost never measured, and in the one recent trial that did measure it, in children, salivary melatonin did not change while the sleep phase moved a little earlier.

There is a gap between "the mechanism exists" and "this product puts the mechanism to work". That gap is exactly where the research on the glasses falls, and we have a separate article about the glasses themselves.

What we think

If you want to do something about your evening light, start with the things whose effect is largest and best measured. Note the order: this is not colour against amount, it is both, with amount first.

Less light first. The dose response curve is the strongest finding in the entire field. And it is not only your screen: leave the lights on in your room until the moment you go to bed and you will fall asleep more slowly than if you had turned them off. At home, late exposure to more than ten lux was enough to produce more awakenings ^[6], and ten lux is just the hallway.

Then colour, which genuinely counts. Of the light that remains, the short wavelength band is what shifts your clock, and the long wavelengths did nothing at all under controlled conditions ^[2]. An amber or red lens removes exactly that part. So it is not an alternative to dimming; it is what is left to do when the screen has to stay on.

Timing over everything. Mistimed exposure is what the systematic review names as the core problem ^[3]. The same light is useful in the morning and harmful at night, and that is not a detail of your rhythm but its definition.

Get outside during the day. In adolescents the pattern across the day was what softened the evening effect ^[8]. This is the cheapest intervention there is and nobody sells it.

And bear in mind that you may be more sensitive, or less, than average, because that difference is large, stable and yours.