Kchj Karin Smolders

A higher illuminance induces alertness even during office hours: Findings on subjective measures, task performance and heart rate measures

Nocturnal white light exposure has shown marked results on subjective and objective indicators of alertness, vitality and mood, yet effects of white light during daytime and under usual office work conditions have not been investigated extensively. The current study employed a mixed-group design (N=32), testing effects of two illuminance levels (200lx or 1000lx at eye level, 4000K) during one hour of morning versus afternoon exposure. In four repeated blocks, subjective reports, objective performance and physiological arousal were measured. Results showed effects of illuminance on subjective alertness and vitality, sustained attention in tasks, and heart rate and heart rate variability. Participants felt less sleepy and more energetic in the high versus the low lighting condition, had shorter reaction times on the psychomotor vigilance task and increased physiological arousal. Effects of illuminance on the subjective measures, as well as those on heart rate were not dependent on time of day or duration of exposure. Performance effects were most pronounced in the morning sessions and towards the end of the one-hour exposure period. The effect on heart rate variability was also most pronounced at the end of the one-hour exposure. The results demonstrate that even under normal, i.e., neither sleep nor light deprived conditions, more intense light can improve feelings of alertness and vitality, as well as objective performance and physiological arousal.

Effects of dynamic lighting on office workers: First results of a field study with monthly alternating settings

Dynamic lighting is designed to have positive effects on well-being and performance. In a field experiment we tested whether these effects are detectable and stable over time when employed in actual work settings. The study consists of two tranches, one following a monthly alternating experimental design, the other a yearly alternating one. This paper reports on the first tranche. In a dual balanced design, office workers experienced dynamic or static lighting according to an a-b-a /b-a-b scheme over three consecutive periods (N = 142, 90, 83). Questionnaire data suggest no significant differences for need for recovery, vitality, alertness, headache and eyestrain, mental health, sleep quality, or subjective performance, although employees were more satisfied with the dynamic lighting. Implications and limitations of the study are discussed.

Shining light on memory: Effects of bright light on working memory performance

This study examined whether diurnal non-image forming (NIF) effects of illuminance level on cognitive task performance depend on task difficulty and time of day. We employed a balanced crossover design with two 60-min sessions of 200 vs. 1000 lux at eye level. Digit-span task difficulty was manipulated within subjects (forward (FDST) vs. backward (BDST) digit-span task), n-back task difficulty was manipulated between subjects (n=1, 2, or 3). Bright light exposure improved FDST performance during the final measurement block, especially in the afternoon. In contrast, BDST performance deteriorated slightly under bright light in the afternoon. Two-back performance was significantly worse under bright light in the afternoon, while no effect of illuminance level was found on 3-back performance. Thus, the more difficult BDST was affected differently by light intensity as compared to the easier FDST. N-back accuracy, however, did not confirm this role of task difficulty. Future studies should investigate whether similar results hold for other types of tasks and how other variables (e.g., time of day, physiological arousal, or other task characteristics) may influence the direction and magnitude of NIF effects on performance.

Non-image forming effects of illuminance level:exploring parallel effects on physiological arousal and task performance

This study investigated diurnal non-image forming (NIF) effects of illuminance level on physiological arousal in parallel to NIF effects on vigilance and working memory performance. We employed a counterbalanced within-subjects design in which thirty-nine participants (mean age=21.2; SD=2.1; 11 male) completed three 90-min sessions (165 vs. 600lx vs. 1700lx at eye level) either in the morning (N=18) or afternoon (N=21). During each session, participants completed four measurement blocks (incl. one baseline block) each consisting of a 10-min Psychomotor Vigilance Task (PVT) and a Backwards Digit-Span Task (BDST) including easy trials (4-6 digits) and difficult trials (7-8 digits). Heart rate (HR), skin conductance level (SCL) and systolic blood pressure (SBP) were measured continuously. The results revealed significant improvements in performance on the BDST difficult trials under 1700lx vs. 165lx (p=0.01), while illuminance level did not affect performance on the PVT and BDST easy trials. Illuminance level impacted HR and SCL, but not SBP. In the afternoon sessions, HR was significantly higher under 1700lx vs. 165lx during PVT performance (p=0.05), while during BDST performance, HR was only slightly higher under 600 vs. 165lx (p=0.06). SCL was significantly higher under 1700lx vs. 165lx during performance on BDST easy trials (p=0.02) and showed similar, but nonsignificant trends during the PVT and BDST difficult trials. Although both physiology and performance were affected by illuminance level, no consistent pattern emerged with respect to parallel changes in physiology and performance. Rather, physiology and performance seemed to be affected independently, via unique pathways.

Higher light intensity induces modulations in brain activity even during regular daytime working hours

We investigated the effect of exposure to bright white light as compared to a commonly experienced illuminance (1000 lx vs. 200 lx at eye level, 4000 K) on electroencephalography spectral power density during daytime. Spectral power density was measured during one hour of exposure in the morning and in the afternoon. Results showed a lower relative power density in the theta range under bright light. In the morning, relative alpha power was also lower under exposure to 1000 lx. The current findings extend earlier results on the effect of illuminance on alertness and arousal in the late evening and at night. Moreover, they largely corroborate results on subjective experience and sustained attention during daytime, and together suggest higher alertness under brighter light even for daytime exposure in everyday situations.