“Circadian lighting” and “human-centric lighting” are now marketing terms attached to a great deal of hardware, some of which does something useful and some of which does very little. The underlying science is real and interesting; the domestic claims made on top of it are frequently overstated.
Here is what is reasonably established, what is not, and what is worth doing at home.
The science, in brief
Human beings have an internal clock of roughly 24 hours, and light is the dominant signal keeping it aligned with the outside world.
Beyond the rods and cones we use for vision, the eye contains cells — intrinsically photosensitive retinal ganglion cells, containing a pigment called melanopsin — that report light levels to the brain's clock rather than contributing to the image we see. Their discovery, relatively recently, is what opened up this field.
These cells are most sensitive to short-wavelength light, in the blue region around 480 nanometres, and they respond to overall intensity as well as spectrum. Light in the morning tends to advance the clock; light late in the evening tends to delay it and suppresses the release of melatonin, the hormone associated with sleep onset.
That much is well supported. The practical consequences at domestic light levels are where it gets more complicated.
Intensity matters more than most products admit
This is the most important caveat and the one most often glossed over.
The circadian system responds to a great deal more light than we use indoors. Outdoor daylight is in the tens of thousands of lux, an overcast day still several thousand. A well-lit domestic room is perhaps 100 to 300 lux, measured at the eye.
So the strongest circadian intervention available is not a bulb. It is going outside in the morning, or sitting near a window, which delivers an order of magnitude more stimulus than any domestic fitting.
It also means claims that a bulb will reset your body clock should be read carefully. A tuneable bulb at ordinary domestic brightness provides a modest signal at best in the morning. Its more useful role is in the evening, where the goal is to provide less stimulus — and reducing light is something a bulb does very well.
What the evidence supports
Bright light in the morning helps. Reasonably well established, particularly for people with delayed sleep timing, shift workers, and in winter at high latitudes. Daylight is best; if you use a lamp, it needs to be genuinely bright and used consistently.
Dim light in the evening helps. Well supported, and the mechanism is straightforward. Note that overall dimness matters at least as much as removing blue — a bright warm room is a stronger circadian signal than a dim cool one. This is why blue-light filtering while keeping the lights bright is a weaker intervention than simply dimming everything.
Consistency helps. A regular light-dark pattern is more valuable than any individual clever intervention, which is precisely where automation contributes.
Darkness at night helps. Even modest light during sleep has measurable effects in studies, so blackout and covering indicator lights is worth more than it sounds.
What is weaker than claimed
That domestic tuneable bulbs replicate daylight. They shift colour temperature within a modest intensity range. Useful, and not equivalent to daylight.
That blue-light glasses and screen filters solve evening light exposure. The evidence is mixed and the effect sizes modest. Reducing overall screen brightness and, more importantly, screen time close to bed appears to matter more, and the behavioural effect of engaging content may exceed the photic effect entirely.
That specific commercial “circadian” curves are optimised for you. They are reasonable generic curves. Individual chronotype — whether you are naturally a morning or evening person — varies substantially and no bulb knows yours.
That colour temperature alone tells you the circadian effect. Two bulbs at the same kelvin figure can have quite different amounts of energy near 480 nm depending on the LED and phosphor used. The metric researchers use is melanopic equivalent daylight illuminance, and essentially no consumer product publishes it. So the honest position is that you cannot compare products properly from the information available.
A sensible domestic setup
Given all that, here is what we would actually do — and note how much of it involves no special hardware.
Morning. Get outside, or sit by a window, for fifteen or twenty minutes. If that is impossible, bright cool artificial light at 4,000 K or above, close to you rather than across the room, since illuminance falls off sharply with distance. A dawn-simulation ramp on a bedside light before your alarm has reasonable support for making waking easier, especially in winter.
Daytime. As much daylight as you can arrange. Work near a window if you can, and take a walk at lunchtime, which does more than any lamp.
Late afternoon. Begin reducing colour temperature and brightness. Gradual is better than a step change, and automation is what makes it happen without thought.
Evening. Warm and dim — 2,200 K or lower, low output, and lamps at low level rather than a bright ceiling fitting, since light from below is less circadian-stimulating than light from above where the sky is.
Night. Genuinely dark, with covered indicator lights. Very dim warm light for any night-time trip, ideally deep amber at a few per cent.
The single most valuable part of that list is the daylight, and it is free.
Where it matters more than average
Shift workers, for whom light timing is a genuine occupational health issue, and where the intervention has to be designed around the rota rather than the clock. This is a case for proper advice.
High latitudes in winter, where morning daylight is simply unavailable for months and bright artificial light in the morning is a reasonable substitute.
Older adults, since the ageing eye transmits less short-wavelength light and lens changes reduce circadian stimulus, which is one plausible contributor to disrupted sleep in later life. Brighter daytime light appears to help.
People with delayed sleep phase, where morning light and evening dimness are part of standard behavioural approaches.
What to buy, if anything
- Nothing, first. Spend a fortnight getting outside in the morning and dimming the lights after nine. This is the largest available effect and it costs nothing.
- Tuneable white bulbs with high CRI in the rooms you use in the evening, so brightness and colour temperature can fall together.
- Table and floor lamps rather than bright ceiling fittings for evening use.
- Blackout and tape for the bedroom. Cheap, and the best-evidenced item on this list after daylight.
- A dawn-simulation light if winter mornings are difficult.
- A very dim warm night light on the route to the bathroom.
- Automation to make it consistent, which is the part technology genuinely contributes.
Our honest summary: the science is real, the domestic hardware helps mainly in the evening, and the biggest thing you can do about your circadian rhythm involves opening a door and standing outside.