The clock sensor
Your eyes have a clock sensor tuned to blue
The third light sensor in the eye, how it was found, and why blue light matters most to it.
17 studies•Nikki Neisler•Reviewed 11 October 2026
The Science of
Light, Screens & Sleep
What light does to your body clock, your sleep and your eyes. One question per article, every study linked to its paper.
- 2 articles
- 20 studies cited
- Last checked 11 October 2026
Light is the strongest signal your body clock gets. A sensor at the back of your eye, tuned to blue, tells it what time it is, and at night the screen in front of you is part of that signal. Decades of research show what evening light does: it holds back melatonin, pushes the clock later and keeps you wired when you should be winding down.
We built Sundown to take that light off your Mac. Each article here takes one question, reads the studies at the level of the paper and links every one of them, so you can see the science for yourself.
Sundown, tested
Tested by Dr. Alexander Wunsch
Sundown eliminates blue light.
Dr. Alexander Wunsch, a German physician who specialises in light and health, measured a Mac screen across the whole blue band, 380 to 500 nm, with Sundown off and at deep warmth. At deep warmth, the blue is gone.
Sundown removes screen flicker.
Flicker-free mode holds the backlight at full power and does the dimming in software. Jack Richards, who makes Sundown, put the screen on an oscilloscope to check. No flicker.
The articles
How we read the research
Every study is read on PubMed or the journal’s own site, at least at the level of its abstract, before a sentence cites it. Each article lists every study it uses with its design, its sample size and what it found, so you can check our reading against the paper.
Spot a study we missed or a sentence that overstates a paper? Write to support@trysundown.com and we will fix it.
What we built from it
The research points at two levers in the hours before bed: less blue light, and less light overall. Sundown pulls both. It warms your Mac’s display down to a deep red and dims it, without asking you to stop using your computer. Tested by Dr. Alexander Wunsch, it eliminates blue light.
Notes on the research
- Sundown's results are its own test. The papers in this library tested light, lenses, lamps and other screens. Sundown was tested by Dr. Alexander Wunsch (blue light) and on an oscilloscope (flicker); the method is on the How we tested page.
- The Night Shift results are mixed. A double-blind phone trial found no significant change in blood melatonin. On an iPad, both Night Shift settings still suppressed melatonin and did not differ significantly from each other. On an iPhone, sleep was no better with Night Shift on than off.Heo et al., 2017 ↗Nagare et al., 2019 ↗Duraccio et al., 2021 ↗
- Eye strain is mostly focusing and blinking. Screen eye strain comes mostly from focusing and from blinking less, and blue-blocking lenses did not reduce it in the trials.Rosenfield, 2011 ↗Singh et al., 2021 ↗Singh et al., 2023 ↗
- Flicker research is mostly about lighting. The symptom evidence comes from office fluorescent lights, not laptops. For temporal dithering, a PubMed search in October 2026 found no peer-reviewed clinical studies of discomfort, eye strain, headache, visual fatigue or migraine.Wilkins et al., 1989 ↗Brodaczewski and Brodaczewski, 2026 ↗
- Screen light has not been shown to harm skin. A systematic review and measurements of everyday screens found no skin hazard from device light.Ceresnie et al., 2023 ↗Charoenpipatsin et al., 2025 ↗
- We make Sundown. That makes us an interested party, which is why every claim here links to its paper. Sundown is a display utility, not a medical device: it does not diagnose or treat insomnia, migraine, eye strain or any other condition.