Abstract
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•Short-wavelength light during nighttime and over a long period (∼6h) can sustain inducing alertness, while long-wavelength (red) light at a shorter duration (∼1h) and lower intensity can almost produce this effect during the daytime until night.•EEG measurements of alertness reveal high activity at different times during exposure to short-wavelength and long-wavelength light, and this increase does not seem to associate with improving subjective and objective measures of alertness.•Rather than melanopic illuminance, L-cone-opic illuminance could be used to determine particular alerting effects of light, such as neural activity in the brain at specified times.•For producing alertness during nighttime, it is recommended to substitute light sources enriched at short-wavelength and middle-wavelength (420–555 nm) with long-wavelength (620 – 640 nm) light to avoid disrupting melatonin secretion.
Light is detected in the eye by three classes of photoreceptors (rods, cones, and intrinsically photosensitive retinal ganglion cells (ipRGCs)) that are each optimized for a specific function and express a particular light-detecting photopigment.
The significance of short-wavelength light and ipRGC's role in improving alertness has been well-established; however, few reviews have been undertaken to assess the other wavelengths' effects regarding timing and intensity. This study aims to evaluate the impact of different narrowband light wavelengths on subjective and objective alertness among the 36 studies included in this systematic review, 17 of which were meta-analyzed.
Short-wavelength light (∼460 – 480 nm) significantly improves subjective alertness, cognitive function, and neurological brain activities at night, even for a sustained period (∼6h) (0.4 < Hedges's g < 0.6, p < 0.05), but almost not during the day when melatonin level is lowest. Long-wavelength light (∼600 – 640 nm) has little effect at night, but significantly increases several measures of alertness at lower irradiance during the daytime (∼1h), particularly when there is homeostatic sleep drive (0.5 < Hedges's g < 0.8, p < 0.05). The results further suggest that melanopic illuminance may not always be sufficient to measure the alerting effect of light.