Lux is the SI unit of illuminance, defined as one lumen per square metre of surface area. It quantifies the light intensity reaching a surface, distinct from lumens, which measure total light output at the source. In sleep biology, lux thresholds determine whether a light exposure is sufficient to suppress melatonin and shift the circadian clock.
Lux measures illuminance: the luminous flux arriving at a surface per unit area, expressed in lumens per square metre. The distinction from lumens matters in practice: a single bulb may emit 800 lumens in all directions (total output), yet deliver only 50 lux to a desk one metre away if the light is diffused across a large area. Lux therefore describes what a surface receives, not what a source produces.
Circadian photoreception is mediated primarily by melanopsin in intrinsically photosensitive retinal ganglion cells (ipRGCs), a class of photoreceptor with peak sensitivity near 480 nm, distinct from the rod and cone pathways that serve conscious vision 3. Melatonin suppression follows a sigmoidal dose-response curve: half-maximal suppression occurs at approximately 100 lux of polychromatic white light, with responses saturating between 1,000 and 10,000 lux 2. Ordinary indoor lighting sits precisely at the steepest point of the suppression curve.
Standard photopic lux is calibrated to the human eye's peak daytime sensitivity at 555 nm, not the 480 nm peak of the circadian system. Melanopic illuminance, which weights lux by melanopsin's spectral sensitivity, consistently outperforms photopic lux as a predictor of melatonin suppression across nineteen laboratory studies 5. The practical implication: two rooms with identical photopic lux readings can have markedly different circadian effects if one uses warm-toned sources (low 480 nm content) and the other uses cool-white or daylight-spectrum lighting.
A professional who works evenings at a desk lit to 300 lux transitions, two hours before sleep, to a room dimmed with warm-toned bulbs to below 10 melanopic lux. This falls well short of the 100 photopic lux that triggers half-maximal melatonin suppression, preserving melatonin onset timing. The same illuminance from a cool-white source, richer in 480 nm light, would carry far greater circadian disruption.
Photopic lux alone does not predict circadian impact: melanopic lux, weighted to melanopsin's 480 nm peak sensitivity, is the operationally superior metric.
Lewy et al. established that human melatonin secretion is suppressible by bright artificial light at 2,500 lux, a threshold vastly higher than ordinary room lighting 1. That finding anchored two decades of research, which gradually revised the suppression threshold downward. Zeitzer et al. subsequently confirmed that the human circadian system is far more sensitive to ordinary indoor light than originally assumed: 100 lux, the illuminance of a typical sitting room, produces approximately half the maximum phase-shifting effect on the circadian pacemaker 2.
Chang et al. demonstrated the clinical consequences: reading on a light-emitting device for four hours before bed, at approximately 30-100 lux, suppressed melatonin by 55%, delayed the circadian clock by more than 1.5 hours, and reduced next-morning alertness compared with reading a printed book 4. For practitioners applying light-management protocols, this means that effective circadian hygiene requires controlling both lux intensity and the spectral composition of light sources, particularly in the 480 nm range where melanopsin sensitivity peaks.
The two units measure different things. Lumens quantify a source's total light output, while lux quantifies the light that actually arrives at a surface per unit area (lumens per square metre). A lamp emitting 800 lumens may yield only 40 lux at a desk if the light disperses across a wide area.
Half-maximal melatonin suppression occurs at approximately 100 lux of white light, the illuminance of a typical domestic room. At 2,500 lux, suppression is near-complete. The dose-response curve is sigmoidal, which means ordinary indoor lighting is not safe from a circadian standpoint: it sits at the steepest part of the suppression response.
Light-therapy devices for seasonal and circadian phase disorders typically deliver 10,000 lux for 20-30 minutes in the morning. This dose follows from the original demonstration that 2,500 lux suppresses nocturnal melatonin in humans, a threshold that anchored subsequent clinical protocol development. The morning timing is critical because light at this intensity also advances the circadian phase.
Screen light affects melatonin even at low lux because digital displays emit strongly in the 480 nm wavelength range where melanopsin is most sensitive. Chang et al. found that four hours of pre-bed screen use at roughly 30-100 lux suppressed melatonin by 55% and delayed the circadian clock by more than 1.5 hours.
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