A new study led by scientists at Penn State found that white light can impact how people experience indoor temperatures. Although the wavelength differences are invisible to the human eyes, they can influence the gap between perceived and real temperature by as much as 1.3 degrees Fahrenheit (F). Credit: Courtesy of Julian Wang / Penn State
The Light Looked the Same. The Room Didn’t Feel the Same.
In A Nutshell
- A Penn State study found that tweaking the invisible wavelength mix inside ordinary white light changed how warm or cold a room felt to people, even though nobody could see any difference in the light.
- Blue-enriched light let people tolerate warmer air before asking for relief, while red-enriched light made people report feeling warmer as a room cooled, two separate effects rather than one.
- Building energy simulations suggest a small thermostat adjustment paired with this kind of lighting could cut cooling energy use by 5.2 to 11.4 percent, though that number comes from a computer model, not a real building.
- The study only involved ten young adults tested in a lab during winter, so it’s an early, exploratory finding rather than a ready-to-use fix for offices or homes.
A person sits in an office where the light overhead looks exactly the same as it did yesterday, yet somehow the room feels warmer and nobody touched the thermostat. A new study from Penn State researchers points to something the eye can’t catch: a subtle shift in the mix of wavelengths making up white light.
Researchers tested two light sources that looked identical to the eye but were engineered with very different blends of blue and red wavelengths underneath. As the room warmed, blue-enriched light changed how long people waited before asking for relief. As the room cooled, red-enriched light changed how warm people said they felt. The two lights nudged different parts of thermal comfort, not the same one twice.
Published in the journal Energy and Buildings, the results offer some of the first direct evidence that light’s spectral makeup, not just its visible color or brightness, can shift how warm or cold a room feels. The research team, led by Julian Wang, a professor of architectural engineering at Penn State, set out to test whether lighting could stretch the range of temperatures people find comfortable. That distinction matters because buildings burn enormous energy keeping temperatures within a narrow comfort band, and if lighting alone can stretch that band even slightly, the payoff for heating and cooling bills could be substantial.
Two Lamps Looked Identical, Yet the Body Reacted Differently
Researchers, led by Julian Wang at Penn State, built two “metameric” light sources, a term for lights that are spectrally distinct but appear visually identical. Both lamps produced ordinary-looking white light at the same brightness, but one shifted more of its output toward blue wavelengths and the other toward red. Every participant confirmed afterward they never noticed a difference in how the room looked.
Ten adults, five men and five women between 18 and 35, took part inside a windowless chamber built to resemble a small office cubicle at Penn State’s University Park campus. Each visited twice, once under blue-enriched light and once under red-enriched light, always at the same time of day. After 30 minutes to adjust at a starting temperature around 76 degrees Fahrenheit, the room’s air temperature slowly rose or fell while participants worked, read, or did puzzles. Every five minutes, they rated how comfortable and how warm or cold they felt, and they had a button to press the moment the room became too uncomfortable. The tightly controlled setup let researchers isolate light’s spectral effects from other factors, like brightness or visible color, that muddied earlier research.
Blue Light Widened Heat Tolerance, Red Light Changed Thermal Comfort in the Cold
When the room slowly heated up, participants under blue-enriched light held out significantly longer before pressing the button, tolerating an extra 0.7 degrees Celsius, about 1.3 degrees Fahrenheit, of warmth before deciding the room needed to cool down, a gap that held up statistically after accounting for body mass and core temperature. Curiously, participants did not consciously report feeling any cooler, and their comfort and warmth ratings did not differ meaningfully between the two lighting conditions. The cooling effect showed up only in what they did, not what they said, suggesting behavior and self-reported feeling don’t always move together.
That pattern flipped once the room cooled instead of heating up. Red-enriched light made people report feeling significantly warmer than they did under blue light, even though the air temperature was identical, but the two light types produced no meaningful difference in how long people waited before signaling discomfort as the room grew cold.
Researchers suspect this split may point to different pathways in the body, one that governs conscious sensation and one that governs behavior, though the study itself doesn’t pin down why behavior changed during warming while self-reported sensation changed during cooling.
A Half-Degree Shift Could Widen Thermal Comfort and Cut Cooling Costs
To translate the lab findings into real-world terms, researchers ran building energy simulations across several U.S. climate zones, modeling a thermostat allowed to drift half a degree Celsius wider, a somewhat smaller adjustment than the 0.7-degree shift observed under blue-enriched light. Cooling energy savings ranged from 5.2 percent to 11.4 percent depending on climate zone. Those numbers come from a computer model, not a test in an occupied building.
HVAC systems are typically one of the largest single draws on a building’s energy budget, and people spend roughly 90 percent of their time indoors, so small shifts in demand add up fast across an office or school. Rather than replacing thermostats with light bulbs, the goal is pairing the two: a building’s lighting could nudge occupants’ comfort just enough to let HVAC systems relax their setpoints without anyone feeling worse off.
None of this requires occupants to notice anything different about their lighting, which may be the most useful part of the finding. It’s still early: this was one small lab study, not a test in real offices, so any building using spectral lighting to trim energy bills remains a ways off. But the idea itself, that light could quietly widen how warm or cold a room feels, is now backed by a real experiment rather than a hunch.
Disclaimer: This article summarizes findings from a single peer-reviewed study and is intended for general informational purposes. It does not constitute engineering, medical, or building-design advice, and the energy savings described come from computer simulation rather than measurements in an occupied building.
Paper Notes
Limitations
The study relied on a small sample of ten young adults, all tested during winter, which limits how broadly the findings generalize to other age groups, seasons, or climates. The paper’s own statistical power was low given the sample size, and the person operating the room’s thermostat during testing was not blinded to which light was active, both of which leave room for unmeasured bias. Sessions relied on air temperature as a proxy for full operative temperature rather than continuously measuring radiant heat and airflow, and the study did not track ventilation rate or indoor air quality. The researchers describe the work as an early, exploratory step rather than a ready-made building solution, and the reported energy savings come from a computer simulation rather than a test in an occupied building.
Funding and Disclosures
The work was supported by an Institute of Energy and the Environment Seed Grant at Pennsylvania State University. The authors reported no competing financial interests or personal relationships that could have influenced the work.
Publication Details
The study, titled “Effects of metameric light on human subjective and behavioral thermal responses in daytime transient thermal environments,” was authored by Nan Wang, Julian Wang, Jeffrey Mundinger, Anne-Marie Chang, Yanxiao Feng, and Chenshun Chen, of Northwestern University, Pennsylvania State University, and the New Jersey Institute of Technology. It was published in Energy and Buildings, volume 364 (2026), article number 117644. The paper is available via DOI: 10.1016/j.enbuild.2026.117644.
