The evidence
Your eyes have a
third light sensor.
It has nothing to do with vision. It tells your brain whether it is day or night — and it is most sensitive to exactly the light your screen emits most of.
Below is the peer-reviewed research on evening light, melatonin and circadian timing. Every claim links to a real study. So does every limitation — including the places the evidence is thin, and the popular claims it does not support.
How light reaches your clock
Three steps, each independently established in the literature.
- 01
A sensor that isn't for seeing
In 2002, researchers identified a third class of light-sensitive cell in the retina — the intrinsically photosensitive retinal ganglion cell. It does not build images. It reports ambient light directly to the brain’s master clock.1
- 02
It is tuned to blue
Two independent teams mapped which wavelengths suppress melatonin in humans. Both found peak sensitivity in the short-wavelength — blue — region, offset from where the visual system is most sensitive.2,3 In a direct comparison, 460 nm light produced a two-fold greater circadian phase delay than green light of equal photon density.4
What screens do at night
The cleanest experiment on this swapped one variable: a light-emitting device versus a printed book, in the hours before bed. Reading on the device made people take longer to fall asleep, suppressed melatonin, delayed the circadian clock, and left them less alert the next morning.7
The mechanism is not mysterious. A screen is a lamp you hold close to your face, and its white light is built from a strong blue peak — the same region the clock is most sensitive to.2,3,4
What happens when you block it
The headline result, stated exactly as the study reports it.
Ostrin et al., 2017 — 21 adults, two weeks
night-time melatonin — from 16.1 to 25.5 pg/mL (P < 0.01)
objectively measured sleep, by actigraphy (P < 0.001)
Participants wore short-wavelength-blocking glasses for about four hours before bed. Subjective sleep quality improved too (PSQI 5.6 → 3.0).8
Insomnia symptoms
Amber lenses for 2 hours before bed beat clear placebo lenses on insomnia ratings and subjective sleep.10
n=14, randomised crossover
Sleep quality and mood
A randomised trial of amber lenses found improved self-reported sleep quality and mood.9
sleep diaries, no melatonin measured
Delayed sleep phase
In patients with delayed sleep phase disorder, evening blue-blocking glasses advanced sleep onset by around two hours.11
clinical population
The part most companies leave out
What this evidence does not show
We sell a blue-light product, so treat us as an interested party and check the links. Here is what we think the honest reading is.
It does not show blue light causes eye strain
A Cochrane systematic review of randomised trials concluded that blue-light filtering spectacle lenses probably make little or no difference to eye strain from computer use — and found no randomised-trial evidence on melatonin at all for those lenses.14Digital eye strain is real, but it is largely a story about blink rate, focus distance and screen time — not wavelength. Sundown’s case rests on circadian timing, not on curing eye strain.
The melatonin result is not unanimous
One randomised, double-blind crossover study of blue-reduced smartphone use found improvements in sleepiness measures, but the change in serum melatonin was not statistically significant.12 We cite it here because leaving it out would misrepresent the field.
The studies are small, and often subjective
The blue-blocking trials above run from 14 to 21 participants, and several rely on self-reported sleep rather than objective measurement.8,9,10 That is enough to be suggestive. It is not enough to be certain, and anyone quoting these numbers as settled fact — us included — should say so.
Glasses are not a screen filter
Most of this research tests amber lenses, not display software. The shared mechanism is reducing short-wavelength light reaching the eye, and we think that transfer is reasonable — but it is an inference, not a finding. No study on this page tested Sundown.
What we do with all of this
The literature points at one lever: reduce short-wavelength light reaching your eyes in the hours before bed, without making you stop using your computer. Sundown shifts your display’s colour temperature down, well past what a built-in night mode reaches, and dims without the flicker of backlight modulation.
The lower the colour temperature, the less short-wavelength energy the panel emits — which is the same lever every study above is pulling, applied to the screen instead of your face.
On the numbers we quote elsewhere
Percentages about how much blue light a given display setting blocks are engineering measurements of a screen, not clinical findings, and they should never be attributed to the studies on this page. We are publishing our spectral measurement methodology so those figures can be checked the same way these citations can. Until that is up, treat any blocking percentage as a product spec awaiting published method — not as science.
Sundown is a display utility, not a medical device.
References
Every link below was confirmed to resolve on 21 July 2026. If you find one that has rotted, tell us at support@trysundown.com and we will fix it.
- 01
Berson DM, Dunn FA, Takao M (2002). Phototransduction by retinal ganglion cells that set the circadian clock.
Science
Identified a third class of light-sensing cell in the retina — intrinsically photosensitive retinal ganglion cells (ipRGCs) — that signal light directly to the circadian clock, separate from the rods and cones used for vision.
PMID 11834835 ↗ - 02
Brainard GC, et al. (2001). Action spectrum for melatonin regulation in humans: evidence for a novel circadian photoreceptor.
The Journal of Neuroscience
Mapped which wavelengths suppress melatonin in humans. Peak sensitivity falls in the short-wavelength (blue) region, not where the visual system is most sensitive.
PMID 11487664 ↗ - 03
Thapan K, Arendt J, Skene DJ (2001). An action spectrum for melatonin suppression: evidence for a novel non-rod, non-cone photoreceptor system in humans.
The Journal of Physiology
Independently derived a melatonin-suppression action spectrum peaking in the short-wavelength range, confirming a non-rod, non-cone photoreceptor system.
PMID 11507175 ↗ - 04
Lockley SW, Brainard GC, Czeisler CA (2003). High sensitivity of the human circadian melatonin rhythm to resetting by short wavelength light.
The Journal of Clinical Endocrinology & Metabolism
6.5 hours of 460 nm (blue) light produced a two-fold greater circadian phase delay than 555 nm (green) light of equal photon density, and roughly twice the melatonin suppression.
PMID 12970330 ↗ - 05
Zeitzer JM, Dijk DJ, Kronauer RE, Brown EN, Czeisler CA (2000). Sensitivity of the human circadian pacemaker to nocturnal light: melatonin phase resetting and suppression.
The Journal of Physiology
Characterised how strongly the human circadian pacemaker responds to nocturnal light, establishing that ordinary indoor light levels are enough to shift the clock.
PMID 10922269 ↗ - 06
Gooley JJ, et al. (2011). Exposure to room light before bedtime suppresses melatonin onset and shortens melatonin duration in humans.
The Journal of Clinical Endocrinology & Metabolism
In 116 healthy adults, ordinary room light (<200 lux) before bed delayed melatonin onset in 99.0% of individuals and shortened melatonin duration by about 90 minutes. Light during usual sleep hours suppressed melatonin by more than 50% in 85% of trials.
PMID 21193540 ↗ - 07
Chang AM, Aeschbach D, Duffy JF, Czeisler CA (2015). Evening use of light-emitting eReaders negatively affects sleep, circadian timing, and next-morning alertness.
PNAS
Compared with a printed book, reading on a light-emitting device before bed made participants take longer to fall asleep, suppressed melatonin, delayed the circadian clock, and reduced next-morning alertness.
PMID 25535358 ↗ - 08
Ostrin LA, Abbott KS, Queener HM (2017). Attenuation of short wavelengths alters sleep and the ipRGC pupil response.
Ophthalmic & Physiological Optics
21 adults (aged 17–42) wore short-wavelength-blocking glasses for about 4 hours before bed for two weeks. Night-time melatonin rose from 16.1 to 25.5 pg/mL (P < 0.01) — a ~58% increase — and objectively measured sleep duration increased by 24 minutes (P < 0.001).
PMID 28656675 ↗ - 09
Burkhart K, Phelps JR (2009). Amber lenses to block blue light and improve sleep: a randomized trial.
Chronobiology International
A randomised trial of amber blue-blocking lenses worn before bed. Participants reported improved sleep quality and mood. Outcomes were subjective (sleep diaries); the study did not measure melatonin.
PMID 20030543 ↗ - 10
Shechter A, Kim EW, St-Onge MP, Westwood AJ (2018). Blocking nocturnal blue light for insomnia: A randomized controlled trial.
Journal of Psychiatric Research
14 adults with insomnia symptoms wore amber lenses or clear placebo lenses for 2 hours before bed across 7 nights, in a randomised crossover design. Insomnia ratings and subjective sleep measures improved with amber lenses (p < 0.05). A small, largely self-reported study.
PMID 29101797 ↗ - 11
Esaki Y, et al. (2016). Wearing blue light-blocking glasses in the evening advances circadian rhythms in patients with delayed sleep phase disorder.
Chronobiology International
In patients with delayed sleep phase disorder, evening blue-blocking glasses advanced sleep onset by roughly two hours. A clinical population, not healthy sleepers.
PMID 27322730 ↗ - 12
Heo JY, et al. (2017). Effects of smartphone use with and without blue light at night in healthy adults: A randomized, double-blind, cross-over study.
Journal of Psychiatric Research
Blue-light-reduced smartphone use before bed was associated with less sleepiness and fewer commission errors. Notably, the change in serum melatonin was NOT statistically significant.
PMID 28017916 ↗ - 13
Tähkämö L, Partonen T, Pesonen AK (2019). Systematic review of light exposure impact on human circadian rhythm.
Chronobiology International
A systematic review of how light timing, intensity, duration and spectrum shift human circadian rhythms.
PMID 30311830 ↗ - 14
Singh S, et al. (Cochrane Review) (2023). Blue-light filtering spectacle lenses for visual performance, sleep, and macular health in adults.
Cochrane Database of Systematic Reviews
A systematic review of randomised trials of blue-light filtering spectacle lenses. Found these lenses probably make little or no difference to eye strain with computer use, and identified no randomised-trial evidence on melatonin at all.
PMID 37593770 ↗
This page is the canonical source for scientific claims made anywhere on this site. If something we publish elsewhere disagrees with this page, this page is right and the other page is a bug.