Research register
What the studies actually found.
Nine independent studies and reviews of evening light, amber lenses and blue-light filtering — each with the design, sample size and measurements it was published with.
SleepSpec was engineered from the mechanism these studies describe. They tested evening light and blue-light filtering rather than SleepSpec itself — so here they are, reported as published.
How to read this
Evidence is not one thing.
Study design
An uncontrolled before-and-after study shows that something changed over time. A randomised crossover trial compares the same people under two conditions, which rules out far more. Both appear below, each labelled with what it was.
Sample size
These are small trials: 12 to 72 participants, plus two workplace studies of 63 and 67 people. Sample sizes like these show a direction clearly. They are not large enough to fix an exact number.
What was measured
Reported sleep and instrument-measured sleep do not always agree. Every entry names its measurement channels, so you can see which is which.
- Melatonin
- Self-reported
Solid: measured by instrument or assay. Dashed: reported by the participants themselves.
The mechanism
Why evening light is the variable.
Evening exposure to short-wavelength, melanopically effective light — including blue and cyan light — can suppress melatonin and, depending on its timing, intensity and duration, delay circadian timing and sleep onset.
Human circadian photoreception is especially sensitive to short-wavelength visible light. The melanopsin photopigment peaks at approximately 479–480 nm, while the standardised melanopic action spectrum at the eye peaks at approximately 490 nm.
Age changes the input. Younger crystalline lenses generally transmit more short-wavelength light to the retina, while ageing causes the lens to become yellower and selectively reduce short-wavelength transmission.
How evening light works, in full →The register
Nine studies, in full.
Grouped by what each one was actually asking. Findings are given in the researchers’ own terms, including the ones that came back negative.
What evening light does
2015
Four hours of light-emitting reading before bed
Reading from a light-emitting e-reader for four hours before bed on five consecutive evenings suppressed melatonin, delayed circadian timing, reduced evening sleepiness and increased sleep-onset latency, compared with reading a printed book.
A controlled laboratory comparison: four hours of screen reading against four hours of print, on five consecutive evenings.
2023
How much melanopic light a display actually delivers
Higher melanopic irradiance reaching the eye from evening displays produced greater melatonin suppression, later melatonin onset and longer sleep-onset latency. The effect tracked the melanopic dose rather than the screen itself.
Measured in healthy men, and about melanopic light reaching the eye rather than any particular pair of glasses.
What happened when people wore filtering lenses
2017
Two weeks of evening short-wavelength blocking
Night-time melatonin increased from 16.1 to 25.5 pg/mL — approximately 58% — and actigraphy-estimated sleep duration increased by approximately 23 minutes after two weeks of evening short-wavelength-blocking glasses.
An uncontrolled pre–post design: all 21 participants wore the glasses, with no comparison group.
2018
Amber against clear, in adults with insomnia symptoms
Amber lenses worn for two hours before bed increased self-reported sleep by approximately 52 minutes and actigraphy-estimated sleep by approximately 29 minutes compared with clear lenses, while also improving insomnia scores and perceived sleep quality.
Fourteen participants, all with insomnia symptoms. Both measures moved, the reported gain being roughly twice the actigraphy-estimated one.
2021
Healthy adults: what people felt, and what the instruments saw
Blue-light-blocking glasses reduced self-reported sleep-onset latency from 24 to 21 minutes and reported awakenings from 2.2 to 1.6 per night. Objective sleep measures did not significantly improve.
Healthy adults rather than people with sleep difficulties, with the reported measures moving further than the objective ones.
2021
Two workplaces, one result that did not repeat
Two experimental studies reported better sleep and improvements in some next-day outcomes, including work engagement and task performance. Stronger effects among later chronotypes appeared in the manager study, but were not replicated in the call-centre study.
Field studies in real workplaces, measured through participant reports and next-day work outcomes rather than sleep instruments.
2021
Third-trimester pregnancy
Melatonin onset occurred approximately 28 minutes earlier with blue-blocking glasses than with partially blocking control glasses. Melatonin onset was calculated for 47 of the participants, and primary sleep outcomes did not significantly improve.
A specific population — third-trimester pregnancy — with melatonin onset calculated for 47 of the 60 participants.
What the reviewers concluded
2021
and later
Two reviews, two verdicts
A 2021 systematic review concluded that there was “substantial evidence” for reduced sleep-onset latency among people with sleep disorders, jet lag or variable shift-work schedules. A later Cochrane review judged the overall sleep evidence mixed and very uncertain.
Two teams weighing the same literature to different standards. Both conclusions are given here.
2025
Twenty-six products on the market, measured
An optical analysis of 26 commercially available glasses described appropriately selected blue-blocking glasses as a potentially low-cost, individualised tool for supporting sleep and circadian health, while emphasising that effectiveness depends on spectral filtering, timing and correct use.
An analysis of lenses rather than a sleep trial. Its three conditions — spectral filtering, timing and correct use — are the ones a buyer can check.
