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.
Most people assume they need to be sleeping under a floodlight for it to matter. They think a dim bedside lamp left on, the glow from a standby light, the street light leaking through the curtains — none of that counts. The science tells a different story.
Your body is detecting light even when your eyes are closed
The biological clock in your brain does not switch off when you fall asleep. It continues monitoring light through the thin tissue of your eyelids. The photoreceptors responsible for circadian timing, particularly the melanopsin-containing retinal cells sensitive to blue-range wavelengths, remain active through the night. Even low levels of light exposure can signal to your brain that morning has arrived before it actually has.
When that signal fires prematurely, your body begins winding down melatonin production and shifting your cortisol and core temperature curves earlier. Over time, this creates a chronic low-grade misalignment between your biological clock and the outside world, a state we now call circadian disruption. For the evidence on what this chronic disruption means for long-term health outcomes, see our piece on could more sunlight help you live longer.
How much light is too much?
Research suggests that exposures as low as 1 to 3 lux of light at night can measurably suppress melatonin. To give that context: a typical bedroom with the curtains partly open on a lit street can easily reach that level. A phone screen on low brightness at arm's length is around 10 lux. A bedside lamp on its lowest setting is often 30 to 50 lux.
Studies have associated habitual light at night with disrupted sleep architecture, reduced melatonin duration, impaired insulin sensitivity, and altered immune function. These are not catastrophic single-night effects. They are the cumulative consequence of months and years of light exposure in hours that should be dark.
The colour of that light matters as much as the amount
Not all wavelengths carry the same biological weight. Blue-range light, peaking around 480nm, is the primary driver of melatonin suppression and circadian clock resetting. This is the wavelength that tells your brain it is midday, and it is present in abundance in standard white LED bulbs, phone screens, tablets, and televisions. Beyond the circadian effects, modern LED lighting has also removed other wavelengths from our daily light diet — see our piece on the hidden health risk in your LED bulbs.
Warm orange and red wavelengths, by contrast, have minimal effect on the circadian system. Our ancestors spent their evenings by firelight, a warm amber glow that posed no threat to the biological night. The LED revolution changed that overnight, in the most literal sense.
What to do about it
The solution is not simply to sit in the dark. It is to ensure that the light in your home after sunset shifts away from blue-rich white light towards amber and red wavelengths. Blackout blinds help with external sources. Covering standby lights matters more than people realise. And switching to circadian-tuned lighting that automatically removes blue wavelengths in the evening addresses the root cause rather than the symptom. For a practical guide to choosing the right light for each room and time of day, see what to ask before you switch on the lights.
The question is not whether low-level light at night affects your health. The evidence on that is clear. The question is whether you are willing to take it seriously before the cumulative toll becomes visible.
