Abstract

The discovery of the third photoreceptors in the retina, intrinsically photosensitive retinal ganglion cells (ipRGCs) revealed that light has non-visual effects such as alertness, core body temperature and hormone suppression. Several studies were conducted to investigate the alerting effects of light with different light intensities. However, most of them used illuminance to represent light intensity, which cannot fully represent responses of ipRGCs.

Moreover, since the agenda of the healthy building is developing, several luminaire manufacturers propose that light products with high correlated colour temperature (CCT) would benefit alertness and circadian entrainment. With research needed to justify this statement, the aim of this study is to examine whether the blue-rich artificial light sources could be an acceptable alternative to daylight, in terms of alerting effects. A new metric for quantifying responses of ipRGCs, Equivalent Melanopic lux (EML) was controlled for both light sources in this study instead of illuminance.

Methodology: Twenty students (male: 9, female: 11) aged 18-35 years were asked to undertake two separate experiments: exposure to daylight (DL); and exposure to artificial lighting (AL). Both daylight and artificial lighting should provide the same level of EML in the eyes. Their subjective alertness was measured by the Karolinska Sleepiness Scale (KSS) and Subjective Vitality Scale (VS). Objective alertness was measured by the Psychomotor Vigilance Test (PVT) and Letter Digits Substitution Test (LDST).

The results showed that when providing the same level of EML, there was no significant difference between DL and AL in subjective sleepiness and overall reaction time. Moreover, exposure to both daylight and artificial lighting can significantly reduce subjective sleepiness and increase vitality in the morning. Nevertheless, AL was considered as brighter and less comfortable than DL.

Results and conclusions: The findings suggest that exposure to light with high EML in the morning can help improve alertness. Artificial lighting with high CCT can provide the same level of alerting stimulus as daylight in the short term, but it’s unsuitable for long- term working. Future studies should consider responses of ipRGCs, as well as rods and cones, to investigate the alerting effects of light.