Short-Term Psychological and Cognitive Responses to Daylight-Simulating Illumination in an Underground Environment: An Exploratory Pilot Field Study

Enclosed underground environments lack natural daylight and may influence occupants’ subjective well-being and performance. This exploratory pilot field study characterized short-term psychological and cognitive responses under two underground environmental conditions: a conventional artificial-lighting environment (Zone 1) and a daylight-simulating lighting environment (Zone 2). Six adult male participants completed a 48-h on-site protocol. All participants spent the first 24 h in Zone 1; during the second 24 h, four participants transitioned to Zone 2, while two remained in Zone 1 as a concurrent reference. Subjective ratings and cognitive tasks were collected at seven time points. For the participant-level analysis, same-time-of-day changes between the first and second day were matched within each participant. All four transition participants showed positive mean matched changes in perceived similarity to natural light, happiness, calmness, relaxation, energy, and restorative exploration, whereas the concurrent-reference participants showed mixed directions. The stress composite changed in the opposite direction for most transition participants, and reaction time and Schulte grid performance showed no consistent condition-related pattern. These findings characterize short-term directional responses in a small pilot sample and provide a basis for larger, counterbalanced studies integrating comprehensive photometric, physiological, and circadian measures.