White Light During Daytime Does Not Improve Alertness in Well-rested Individuals

Abstract

Broad-spectrum light applied during the night has been shown to affect alertness in a dose-dependent manner. The goal of this experiment was to investigate whether a similar relationship could be established for light exposure during daytime. Fifty healthy participants were subjected to a paradigm (0730-1730 h) in which they were intermittently exposed to 1.5 h of dim light (<10 lux) and 1 h of experimental light (24-2000 lux). The same intensity of experimental light was used throughout the day, resulting in groups of 10 subjects per intensity. Alertness was assessed with subjective and multiple objective measures. A significant effect of time of day was found in all parameters of alertness (p < 0.05). Significant dose-response relationships between light intensity and alertness during the day could be determined in a few of the parameters of alertness at some times of the day; however, none survived correction for multiple testing. We conclude that artificial light applied during daytime at intensities up to 2000 lux does not elicit significant improvements in alertness in non-sleep-deprived subjects.

Light effects on circadian and homeostatic regulation: alertness increases independent of time awake

“…When sleep pressure rises further, accumulated sleep pressure overrides positive effects of bright light exposure. This hypothesis is consistent with the observations that alerting effects of light during daytime are hard to determine in wellrested individuals 38 , while significant effects have been reported in sleep deprived individuals 39 . In addition to effects of time awake, there is also circadian variation in subjective alertness, with the largest decrease in alertness at the end of the night, as has been reported in other FD studies 12 .…”

Daytime melatonin and light independently affect human alertness and body temperature

“…Our study did not demonstrate alerting effects of light after placebo ingestion, indicating that light under these circumstance does not induce alertness during daytime, confirming previous observations. 33 This emphasizes differences between night and day, since alerting effects of light during the night have been reported (reviews in Ref. 18,19 ).…”

“…Exposure to light improves alertness during nighttime, when melatonin concentrations are usually high and CBT decreases. [9][10][11]16,[20][21][22][23][24][25] However, several studies report absence of light-induced alertness during daytime, when alertness and CBT levels are high and melatonin is absent, 18,20,[26][27][28][29][30][31][32][33][34][35][36][37][38][39][40] , although other studies report contradictory data. 10,41,42 These paradoxical findings can be reconciled when an alertness ceiling level during daytime is considered, which prevents light to further increase alertness.…”

Changing color and intensity of LED lighting across the day impacts on human circadian physiology, sleep, visual comfort and cognitive performance

“…Observed improvements are probably due to the sensitivity of ipRGCs, as magnetic resonance imaging showed that blue light activates brain regions (prefrontal cortex and thalamus, responsible for alertness and cognition) even in blind individuals with still intact ganglion cells 39 . Therefore, we hypothesized better daytime cognitive performance although other studies also reported contradictory results [40][41][42][43][44][45][46][47][48][49][50][51][52][53][54][55] . It should be noted, that due to the high spectral quality (resulting in a very high CRI) in both lighting conditions, the sLED melanopsin weighted irradiance of 7 μW/cm 2 was already 23% more compared to 5.7 μW/cm 2 if a conventional white LED with a lower CRI at the same illuminance and CCT would have been used.…”