Salāmat-i kār-i Īrān (May 2020)
Non-visual effects of the light source “correlated color temperature” on electroencephalographic spectral power, sleepiness and neurobehavioral performance
Abstract
Background and aims: Fatigue and sleepiness (decreased alertness), in addition to a negative impact on performance and quality of work, are considered as one of the leading causes of human error and accidents in work environments. There is much evidence indicating that physical factors in the workplace could affect fatigue, vitality, motivation, and productivity of individuals. A physical environment suitable for activity is formed by various factors, among which light is known as an essential element. Recent photobiological advances and recognition of intrinsically photosensitive retinal ganglion cells (ipRGCs) have shown that in addition to improving eyesight, light can affect the circadian and homeostatic regulations and melatonin suppression in human. Furthermore, light can have acute effects on the physiological, psychological, neurobehavioral, and neuroendocrine responses, such as improvement in the alertness and neurobehavioral performance, which are known as the non-visual or non-imaging forming effects (NIF) of light. However, the possible roles of these potential effects in the improvement of human safety and efficiency have not been thoroughly investigated. Some studies have shown that monochromatic blue light or blue-enriched white light (high correlated color temperature white light or high CCT) can enhance levels of alertness, and improve mood and cognitive function. Although recent evidence has indicated that monochromatic red light or red saturated white light (low correlated color temperature white light or low CCT) has also been able to induce such positive effects. There is still an open question left unanswered: “which of these lighting conditions (high CCT vs. low CCT) has a stronger effect on the alertness level and neurobehavioral function?” Therefore, the present study tested this hypothesis in a simulated office workplace environment with the recommended illumination level of 500 lx on the desk for daytime office work environments during the morning hours. Methods: In this study, 20 healthy paid volunteers (male; mean ± SD age, 27.6 ± 3.6 years) were selected and followed the experimental protocol. All participants were interviewed about the quality of sleep, lifestyle habits, and general health. The inclusion criteria were: not having any mental or physical health problem, having a good sleep quality according to the Persian version of Pittsburgh Sleep Quality Index (PSQI score<5 ), being neither extreme early nor extreme chronotype, having a regular sleep-wake state (bedtimes 22:00 to 24:00 p.m. and wake up between 07:00 and 08:00 a.m.), not smoking, not traveling to a different time zone or experiencing shift-work during three months prior the experiment, no history of eye diseases, and having a normal color blindness as evaluated by the Ishihara test. To control the effect of potential differences in the levels of alertness due to circadian variations and sleep pressure between the light conditions, the participants completed a sleep/wake log, starting one week prior to the beginning of the study. In addition, they were asked to keep a regular sleep/wake state during the study. The participants were also asked not to drink caffeine and/or alcohol about 12 h before the experiment. The aim of the study was described for all participants and they signed an informed consent before the commencement of the study. In addition, the protocol of the study was confirmed by the university ethics committee. The present study had a repeated-measures design, and the participants were exposed to four light conditions for 140 minutes in a counterbalanced order with a one-week interval. The light conditions were dim light (DL, <5 lx, control), and a 500 lx light intensity on the desk level for high CCT white light (HWL, nominal CCT= 12000 K), low CCT white light (LWL, nominal CCT= 2700 K), and standard white light (SWL, nominal CCT= 4000 K).The study was performed in an air-conditioned room with an area of 19 m2. The room’s windows were closed with light-blocking curtains to restrict the penetration of daylight into the experimental setting. Electroencephalogram (EEG) activity (5–7 Hz: theta, 5-9: alpha-theta, 8–12 Hz: alpha, and 13–30 Hz: beta), subjective sleepiness (Karolinska Sleepiness Scale, KSS), subjective mood (Visual Analogue Mood Scale, VAMS), cognitive performance tests (sustained attention, working memory, selective attention task, and inhibitory capacity) and subjective evaluation and beliefs of the participants about the light conditions were measured. The data were analyzed using the MATLAB software package (ver. R2012a, Math-Works, USA) and version 20.0 of the SPSS software (IBM, Armonk, NY, USA). Repeated-measures analysis of variance (ANOVA) was conducted, and where necessary, the Greenhouse–Geisser correction was applied. A 4 (light conditions) × 6 time intervals ANOVA was performed for the EEG activity measures (alpha, theta, beta, and alpha-theta) and CPT data. For the subjective sleepiness and mood, a 4 (light conditions) × 3 time intervals ANOVA was performed. Also, a 4 (light conditions) ANOVA was performed using each cognitive performance (GO/NO-GO, 2-Back, and divided attention) outcome measures. A 3 (light conditions) ANOVA was performed using each subjective evaluation and belief measures about the light conditions. The Bonferroni-adjusted post-hoc tests were applied to multiple comparisons (P<0.05). Results: The means±standard error (SE) of the normalized alpha power was 1.083±0.018 for HWL, 1.147±0.021 for SWL, 1.069±0.021 for LWL, and 1.225±0.027 for DL condition. The Bonferroni-adjusted post-hoc tests indicated a significantly lower power under HWL (P=0.012) and LWL (P=0.006) compared to DL condition. The other comparisons revealed no significant differences. The means±SE of the normalized alpha-theta power was 1.009±0.012 for LWL, 1.053±0.014 for SWL, 1.024±0.016 for HWL, and 1.106±0.017 for DL condition. Post-hoc tests showed a significantly lower alpha-theta power under LWL (P<0.001) and HWL (P=0.033) compared to DL condition. The other comparisons indicated no significant differences. No significant main effect of the light conditions and interaction between them and the time intervals in the normalized beta and theta powers were observed. The means±SE of the normalized subjective sleepiness were 1.129± 0.045 for LWL, 1.25±0.057 for SWL, 1.127±0.052 for HWL, and 1.499±0.074 for DL condition. Post-hoc tests showed that HWL (P=0.002) and LWL (P=0.001) conditions significantly decreased the sleepiness compared to the DL condition. The other comparisons revealed no significant differences. Furthermore, the present study results indicated that the means±SE of the normalized mood scores were 1.048±0.013 for SWL, 1.066±0.011 for HWL, 0.942±0.014 for DL, and 1.158±0.02 for LWL condition. Post-hoc tests indicated that the participants had significantly better mood under the LWL (P<0.001), SWL (P=0.001), and HWL (P<0.001) conditions compared to the DL condition. Furthermore, the LWL enhanced the participantschr('39') mood state as compared to the SWL (P<0.001) and HWL (P=0.009) conditions. The means±SE of the normalized mean reaction time for continuous performance test (CPT) were 0.998±0.011 for SWL, 0.977±0.011 for HWL, 1.066±0.017 for DL, and 0.985±0.011 for LWL condition. The post‐hoc with Bonferroni‐adjusted pairwise comparison revealed a significantly lower mean reaction time under HWL (P<0.001), LWL (P=0.009), and SWL (P=0.026) conditions compared to the DL condition. Furthermore, the means±SE of the normalized mean reaction time for GO/NO-GO task were 305.13±13.959 ms for HWL, 308.91±13.78 ms for SWL, 304.64±14.11 ms for LWL, and 327.49±17.7 ms for DL condition. Post-hoc tests indicated a significantly lower mean reaction time under LWL (P=0.033) and HWL (P=0.034) conditions compared to DL condition. The means±SE of the normalized mean reaction time for 2–Back task were 394.24±32.22 ms for SWL, 387.92±31.64 ms for HWL, 398.62±31.81 ms for DL, and 384.16±30.66 ms for LWL condition. Post-hoc tests revealed a significantly lower reaction time under LWL (P=0.001) and HWL (P=0.027) conditions compared to DL condition. The means±SE of the normalized mean reaction time for selective attention task were 4.16.25±10.262 ms for SWL, 415.95±10.292 ms for HWL, 408.05±10.75 ms for LWL, and 437.75±9.618 ms for DL condition. Post-hoc tests showed a significantly lower reaction time under LWL (P=0.001), HWL (P=0.027), and SWL (P=0.02) conditions compared to DL condition. The Bonferroni-adjusted post-hoc tests did not show any significant differences between HWL, LWL, and SWL light conditions in the CPT, GO/NO-GO, selective attention, and 2–Back tasks. The participants believed that there was no significant difference between the light conditions (SWL, LWL, and HWL) according to the subjective appraisals of light, including brightness, distribution, activating, adequacy amount, and color. Also, the participants reported that the light conditions did not significantly improve their performance. In contrast, the volunteers stated that the LWL (P=0.006) condition was more effective in improving their mood status compared to the SWL condition. Also, about the pleasantness of light, the participants preferred the LWL (P=0.037) over the HWL condition. Conclusion: Under natural conditions (healthy participants and with regular sleep-wake cycle), both the low and high CCT lights (500 lx at the desk) improved alertness and performance compared to the DL condition during the morning hours. In contrast, compared to the SWL, no significant improvement in alertness and cognitive performance during inhibitory capacity, working memory, selective attention, and sustained attention tasks was observed. Briefly, it can be concluded that in addition to the relative preferences of the LWL (2700 K) light condition by the participants, it has had a significant impact on improving the mood of the participants. Hence, the designing and application of lighting interventions by using low correlated color temperature lighting sources can be beneficial for reducing fatigue and sleepiness and improving performance and mood during the morning hours although more studies are required to determine the optimal parameters for lighting interventions.
