Effects of light exposure on human health in buildings
Over the past three years, our Focus Group has explored the pathways between light exposure and cognition, circadian entrainment, and thermal physiology in the built environment. Experimental and applied simulation-based approaches have helped to build a holistic understanding of human physiology and comfort and develop new models to inform lighting design strategies and maintain healthy circadian rhythms.
Focus Group: Human-Centric Building Performance
Prof. Siobhan Rockcastle (University of Oregon), Alumna Hans Fischer Fellow (funded by the TUM Georg Nemetschek Institute)
Bilge Kobas (TUM), Doctoral Candidate
Host: Prof. Thomas Auer (TUM)
Figure 1
During this Fellowship, the team engaged in both experimental and simulation-based research to explore the effects of indoor light exposure on human health indicators and their potential applications to building performance. Our experimental work explored the effects of indoor light exposure on human cognition, mental state, thermal physiology, and melatonin suppression through two experiments. The first experiment took place during the summer of 2023 and utilized the SenseLab to expose 16 participants to a range of indoor lighting conditions during the five hours leading up to their habitual bedtime. Each participant was exposed to one series of lighting conditions during each of the four sessions, with the order of lighting conditions randomized in pairs. Each session was five hours in duration, with a one-hour adaptation period and individual start times adapted to the habitual bedtime of each participant pair. The four lighting conditions included continuous exposure to very dim (1 lx), dim (10 lx), moderately bright (70 lx), or very bright (~600 lx) light (measured at eye level) from an overhead dimmable LED lighting system.
Continuously logged data included environmental variables (lux, temperature, relative humidity, and CO2)) and thermal physiological data such as core body temperature (CBT) from BodyCap ingestible capsules and skin temperature through distal and proximal sensors. Additional data on cognition (n-back and pvt tests), mental state (NASA task-load index, and MAS tense/calmness arousal), sleepiness (Karolinska sleepiness scale), and thermal comfort (ASHRAE 7-point scale) were logged every 30 minutes. Melatonin suppression profiles were also monitored through saliva samples collected every 15 minutes. The team was particularly interested in how the data from these samples and tests varied over the five-hour session. We were able to monitor CBT data into the sleep cycle for each participant from the ingested BodyCap capsules, allowing the research team to see latent effects into the evening.
Key findings of this first experiment include a nonlinear effect of evening light exposure on cognitive performance and mental state across a range of indicators. In terms of cognitive performance, participants showed increased accuracy on n-back tests between the very dim, dim, and moderately bright lighting conditions, but degraded performance between the moderately bright and very bright conditions. Affective state also eroded between the moderately bright and very bright condition, with participants rating their state as more nervous, tense, anxious and stressed under the brighter lighting condition. The nonlinear trend was seen again for indicators of mental and temporal demand, with the NASA task load index showing progressively lower mental demand between each of the first three lighting conditions and a statistically significant increase in demand between the moderately bright and very bright conditions (Fig. 1). Temporal demand followed a similar nonlinear trend. Results from these findings were submitted for publication in Reitmayer at al., 2025. These findings provide evidence to suggest that bright light exposure at night (in day-active people) can erode cognitive performance and mental state rather than provide a positive alerting effect.
Figure 2
Our team is still analyzing the thermal physiology data for publication, but several interesting observations can already be made. The first is that light exposure appears to impact CBT relatively instantaneously (within one hour of exposure), with the very dim condition resulting in an average CBT that is 0.2 °C lower than the average core body temperature recorded under the very bright lighting condition. While Fig. 2 shows a normal circadian rhythm to average CBT over time, participants exposed to the very dim lighting condition in the five hours leading up to bedtime experienced persistently lower core body temperatures during the sleep cycle immediately following their session.
We additionally analyzed melatonin suppression profiles based on each participant’s saliva samples and found an increase in the suppression of average melatonin (pg/mL) between each set of lighting conditions over time. The very dim and dim conditions showed similar suppression curves after 150 minutes, while the moderately bright and very bright conditions showed continued suppression throughout the duration of the session. While the average response curves produced by this data were clean, the research team noticed individual variation between subjects that motivated our second experiment, which was executed in the summer of 2025.
This second experiment examined melatonin suppression and potential carryover effects of repeated evening light exposure using a within-subject design. Nine healthy adults with regular chronotype and good sleep quality completed three consecutive phases: 1) seven-day pre-ambulatory baseline at home, 2) 10–13 day in-laboratory exposure phase, and 3) seven-day post-ambulatory follow-up at home.
Participants were assigned to groups on the basis of habitual bedtime (HBT; 22:00-24:00). During the laboratory phase, subjects arrived 6.5 hours before HBT and spent five hours in a controlled climate chamber before returning home. Sessions were either dim (D: five hours at 10 lux) or bright (B: three hours at 10 lux, 1.5 h at 1000 lux, 0.5 hours at 10 lux). Light exposure sequences followed a counterbalanced De Bruijn design to create all possible triplet combinations.
Saliva samples for melatonin were collected every 30 minutes. Throughout all phases, participants wore continuous glucose monitors, ActiGraph devices (activity/sleep), and ActLumus sensors (light exposure). During laboratory sessions, iButton sensors captured skin temperature and telemetric capsules recorded core body temperature. Psychomotor vigilance tests and questionnaires (sleepiness, mood, comfort) were administered at regular intervals.
Our initial results showed that bright light acutely suppressed melatonin when compared to dim light, with no evidence of carryover effects from prior laboratory light exposure. There were noticeable differences in melatonin suppression between participants, and additional analysis is ongoing to understand these variations. Interpersonal differences in melatonin suppression continue to be underexplored in the literature and have the potential implications in experimental design as well as recommendations for the effects of light exposure on circadian health across a population.
In parallel to this experimental work, Siobhan Rockcastle has used a portion of her academic sabbatical to develop new computational workflows that apply existing models for daytime melanopic light exposure to an annual time series. This applied research is intended to support designers who want to integrate circadian lighting design recommendations into workflows that allow them to evaluate the health potential and energy demand from daylight and electric light sources. This applied research was additionally supported by a grant from the American Institute of Architects and by gift funds provided by the Velux corporation. This work has resulted in one published journal article, two peer-reviewed conference papers, a report and design guide, and one additional journal article that is in preparation. An abstract for this work has been selected to be presented at the annual conference of the American Institute for Architects (AIA) in 2026.
In close collaboration with Prof. Manuel Spitschan (TUM), Amelie Reitmeyer and Letizia Wörrlein (doctoral and master candidate, TUM), Prof. Kelly Johnstone, Prof. Margaret Cook and Dr. Cassie Madigan (all University of Queensland).
Selected publications
- A. Reitmayer, B. Kobas, M. Kammermeier, C. R. Luque, K. R. Johnstone, C. Madigan, M. M Cook, T. Auer, S. F. Rockcastle, M. Spitschan, “Non-linear effects of evening light exposure on cognitive performance,” bioRxiv 2025.03.16.643585. doi: doi.org/10.1101/2025.03.16.643585 [article in revision at Communications Psychology]
- S. F. Rockcastle and A. Mahic, “Simulating the annual energy demand to meet non-visual health recommendations from a luminaire level lighting control system,” Energy and Buildings, vol. 303, p. 113772, Jan. 2024. doi: 10.1016/j.enbuild.2023.113772
S. F. Rockcastle, A. Mahic, and J. Christoffersen, “Evaluating the role of glazing area, orientation, and latitude on circadian health potential in a residential space,” in: Proc. Healthy Buildings Conference 2025, Reykjavik, June 8-11, 2025.
In Preparation:
- B. Kobas, A. Reitmayer, M. Kammermeier, C. Rivera Luque, K. Johnstone, C. Madigan, M. Cook, T. Auer, M. Spitschan, and S. F. Rockcastle, “The effect of indoor light exposure at night on body temperature rhythms in day active people,” Journal of Circadian Rhythms [in preparation].
- S. F. Rockcastle, A. Mahic, D. Gravelle, T. Chrousos, and J. Siple, “An annual climate-based approach to evaluating melanopic illuminance in daylit spaces,” LEUKOS: the journal of the illuminating engineering society [in preparation].