September 11, 2026
When More Daylight Doesn't Mean Better Daylight
Researchers found that a much higher daylight factor still failed to consistently support circadian rhythms
For decades, a 2% daylight factor has functioned as an informal finish line in daylighting design: get that much daylight into a room, and the space counts as adequately lit. The benchmark never carried the force of mandatory U.S. building codes, which regulate window area rather than calculated daylight factors, but it has been a fixture of voluntary green building standards and international codes like Britain's BS 8206-2. New field research published in LEUKOS, The Journal of the Illuminating Engineering Society, suggests even that more modest role may deserve a second look.
Researchers at the Norwegian University of Science and Technology tracked 13 students living in north-facing dormitory units near Trondheim, first under natural daylight conditions averaging a 6.7% daylight factor, more than triple the regulatory minimum, then under conditions reduced to the mandated 2%. Using saliva samples to track dim light melatonin onset, along with sleep, alertness, and vigilance data, the team wanted to know whether either condition supported healthy circadian rhythms. The answer, even at 6.7%, was not consistently.
The finding might be one that should catch the attention of specifiers and building developers alike. Reducing daylight to 2% produced a trend toward later melatonin onset, though the researchers stop short of calling it proof: the aggregate difference was not statistically significant, likely a function of the study's small sample. What they did find was a consistent pattern of insufficient morning light paired with excessive evening light exposure, a combination associated with circadian phase delay.
The Metric May Be Measuring the Wrong Thing
Daylight factor was built to answer a visual question: is there enough light to see and work by. Circadian response depends on something different, the intensity, spectrum, timing and duration of light hitting the eye across a full day, not just its presence in a room. The study's authors argue those two questions have been treated as one for too long, and that daylighting requirements built for visual comfort say little about whether occupants are getting light exposure that keeps their internal clocks on schedule.
For lighting people, this reframes a familiar debate. It's tempting to read the results as an argument for raising the daylight factor threshold. But the researchers found that tripling it still fell short, which points toward a harder problem than tightening a single number.
The paper calls for daylighting standards that account for nonvisual effects directly, likely through metrics tied to melanopic equivalent daylight illuminance rather than daylight factor alone. The Illuminating Engineering Society has already begun exploring such metrics, though how quickly they migrate into recommended practices (RPs) or enforceable code language remains an open question.
The Trondheim study was small, seasonal, and limited to one window orientation, and its authors are careful to say so. But it adds to a growing body of research suggesting that "sufficient daylight" and "biologically useful daylight" are not the same claim. For designers used to hitting a daylight factor number and calling it done, that distinction may be the more consequential story here.
