Blue light explained: How it affects your sleep and body clock

Blue light wavelengths emitted from our phones and devices are linked to poor sleep and circadian disruption.

By far the biggest source of it is the sun. Daylight is full of blue light, which is the whole reason your body uses it as a cue for when to wake and when to rest.

But when it’s emitted from LED bulbs, phone screens, laptops, tablets, and televisions, it means this circadian signal is now abundant at 11pm, in your bedroom, a few inches from your face. That means when you should be winding down for sleep, you’re giving your body a strong signal that it’s time to be alert.

And the consequences are potentially serious.

In this guide, we explain why blue light has been shown to be so disruptive to our sleep, and how you can take action.

Highlights

• Blue light is a normal part of daylight. The problem is getting it in the evening from our screens and devices.
• Your brain uses blue light waves to tell day from night, so evening exposure delays the signal that starts your wind down.
• Ordinary room light is enough. In one study, light under 200 lux delayed melatonin in 99% of people.
• A delayed body clock carries into the next day, which is how one late night becomes several.

How blue light delays your sleep

Your eyes carry a set of receptors that have nothing to do with vision. Alongside rods and cones sits a much rarer type of cell, containing a pigment called melanopsin, wired directly to the part of your brain that keeps time. These cells respond to light  even when every signal from rods and cones is blocked. They exist to report the time of day.

A study exposed 72 people to single wavelengths of light  across 627 melatonin suppression tests and found the blue light wavelength of 446 to 477 nm the most potent.

As light fades in the evening, melatonin rises, and your body begins preparing for sleep: temperature drops, alertness falls, you start to feel tired.

But blue light in the evening suppresses that release, so the wind down never properly starts.

And it takes less light than most people expect. A study of 116 people  found that ordinary room light, under 200 lux, delayed the onset of melatonin in 99% of individuals compared with dim light.

One study compared reading a printed book before bed with reading on a light-emitting e-reader . On the e-reader, people took longer to fall asleep, felt less sleepy in the evening, produced less melatonin and were less alert the next morning.

What our own data shows

Everything above comes from laboratories. We ran a small test in ordinary life.

An Ultrahuman pilot study  followed 13 people wearing a Ring AIR, gathering 1,046 days of data, with each person tracked for three to four months. Eight of them wore blue light-blocking glasses from dusk until bedtime for four weeks. The rest carried on as usual.

In the blue light-blocking glasses group , overnight resting heart rate stayed stable, whereas the other group’s moved around, and their recovery scores declined less steeply.

Only the glasses group held their sleep scores steady, and they kept them even after they stopped wearing the glasses, which is what you would expect if the effect is on the body clock rather than on any individual night.

How to minimise blue light in the evening

• Dim the room before you worry about the screen: Room light under 200 lux is enough to delay melatonin, so turning the lights down does more than filtering one device while the ceiling light stays on.
• Use warm, low lamps after dark: Rather than overhead lighting. Light from below and behind you reaches those receptors less directly than a ceiling light does.
• Turn the brightness down, not just the color: Night mode shifts the wavelengths your clock is most sensitive to, which helps, but a bright screen is still a bright screen.
• Give yourself an hour or two. Evidence on exact timing is thin, so treat the last hour or two before bed as part of your night rather than chasing a precise number.
• Consider blue light-blocking glasses as an addition, not a substitute for dimming. They filter the wavelengths  those cells are tuned to, and they are an evening tool rather than an all-day one.
• Get bright light in the morning: Reset your clock in the morning with light first thing.

Frequently asked questions

Is blue light bad for you?

Not in itself. Daylight is full of it, and that is how your clock stays set. The problem is blue light at the wrong time of day, when it tells your brain the night has not begun.

Does blue light damage your eyes?

This is where most writing on the subject overreaches, including ours. A review of blue light and the eye  confirms blue light can induce photoreceptor damage, but those findings come from laboratory conditions at intensities well above a phone at arm’s length. That ordinary screen use damages human retinas has not been established. The circadian effects are well demonstrated. The retinal risk is not.

Does night mode on my phone work?

It reduces the short-wavelength output your clock is most sensitive to, so it moves in the right direction. It does not make a bright screen dim, and brightness matters too.

Do blue light glasses stop you getting the light you need?

Worn in the evening, no. Worn all day, you would be filtering the signal your clock needs in the morning, which works against you.

References

1. Berson DM, Dunn FA, Takao M. Phototransduction by retinal ganglion cells that set the circadian clock. Science 2002;295(5557):1070-1073. PMID 11834835 . Rat retina.
2. Brainard GC, Hanifin JP, Greeson JM, et al. Action spectrum for melatonin regulation in humans: evidence for a novel circadian photoreceptor. Journal of Neuroscience 2001;21(16):6405-6412. PMID 11487664 . 72 subjects, 627 melatonin suppression tests.
3. Gooley JJ, Chamberlain K, Smith KA, et al. Exposure to room light before bedtime suppresses melatonin onset and shortens melatonin duration in humans. Journal of Clinical Endocrinology and Metabolism 2011;96(3):E463-472. PMID 21193540 . 116 participants.
4. Chang AM, Aeschbach D, Duffy JF, Czeisler CA. Evening use of light-emitting eReaders negatively affects sleep, circadian timing, and next-morning alertness. PNAS 2015;112(4):1232-1237. PMID 25535358
5. Tosini G, Ferguson I, Tsubota K. Effects of blue light on the circadian system and eye physiology. Molecular Vision 2016;22:61-72. PMID 26900325 . Review.
6. Fernandes T, Shanmugam A, Gupta K, et al. Evening blue light exposure, its blocking and influence on individual recovery: an Ultrahuman Ring AIR pilot. Ultrahuman Science . 13 participants, 1,046 days of Ring AIR data.