Dr Martin Moore-Ede is a former Harvard Medical School professor, leading circadian clock researcher, and Chief Health Advisor at LightHealth. He discovered the suprachiasmatic nucleus, the biological clock in the human brain, and has spent over 40 years studying how light governs human health.
When most people choose a light bulb, they ask two questions: how bright is it, and how much does it cost? These are reasonable questions. They are also almost entirely the wrong questions for your health.
Here are the ones that actually matter.
What time of day will this light be on?
This is the single most important question, and it is almost never asked. The biological impact of any light source depends heavily on when in the 24-hour cycle it is active. Bright, blue-rich light in the morning supports circadian entrainment, alertness, and metabolic health. The same light at 10pm suppresses melatonin, delays sleep onset, and disrupts the biological processes that govern overnight repair and recovery. For the evidence on how much even low levels of evening light affect health, see our piece on how much light harms health at night.
A light that is excellent for a kitchen at 8am may be actively harmful in a bedroom at 11pm. Time of use should drive the choice of spectrum.
What is the colour temperature?
Colour temperature is measured in Kelvin. Higher numbers, 5000K and above, indicate cool, blue-rich light that resembles noon daylight and has the strongest effect on the circadian system. Lower numbers, 2700K and below, indicate warm, amber-toned light with minimal circadian disruption.
For most living spaces used in the evening, a colour temperature below 2700K is appropriate. For workspaces and kitchens used primarily in the morning, a higher colour temperature in the 4000 to 5000K range supports alertness and circadian entrainment. Standard LED bulbs sold in the UK are predominantly 2700K or 3000K, adequate for evenings but too warm to deliver a meaningful morning light signal.
Does the spectrum include violet wavelengths?
Almost all standard LED bulbs are built on a blue-pump architecture: a blue LED chip coated in phosphor that converts blue light to white. This produces a spectrum with a residual blue peak and a broad warm band, but virtually no violet light in the 360 to 400nm range.
Violet wavelengths matter for eye health, particularly for children, where violet light deprivation is now associated with accelerated myopia progression. For the full evidence, see our piece on the hidden health risk in your LED bulbs. A full-spectrum light source that extends into the violet band more closely resembles natural outdoor light and supports the biological processes that depend on it.
Can the light dim and shift colour as the day progresses?
A static light source, the same spectrum all day at the same intensity, is a fundamental mismatch with human biology. Natural light changes continuously: bright and cool in the morning, intense at midday, warm and fading in the evening. Each phase carries a different biological signal.
Lighting systems that tune both intensity and colour temperature across the day, shifting from cool bright light in the morning to warm dim light in the evening, deliver something closer to what the circadian system is designed to receive. This is what human centric lighting means in practice.
What does this light mean for the people using it?
Lighting is often specified for spaces rather than people. An office is lit to a lux level that meets workplace guidance. A hospital ward is lit to enable nursing tasks. A bedroom is lit to whatever the previous tenant chose. In almost no context is the question asked: what does this specific light, at this specific time of day, do to the biology of the people in this room?
That question is now answerable. The science exists. The technology exists. The missing ingredient is the habit of asking. For the long-term health evidence on why getting this right matters, see our piece on could more sunlight help you live longer.
