Abstract
The short wavelength, i.e., blue light, is crucial for non-image forming effects such as entrainment of the circadian system in humans. Moreover, many studies showed that blue light enhances alertness and performance in cognitive tasks. However, most scientific reports in this topic are based on experiments using short exposure to blue or blue-enriched light, and only a few focused on the effects of its reduced transmittance, especially in longer periods. The latter could potentially give insight into understanding if age-related sleep problems and cognitive decline are related to less amount of blue light reaching the retina, as the eyes’ lenses yellow with age. In this study, we investigated the effects of prolonged blocking of blue light on cognitive functioning, in particular—sustained attention and visuospatial working memory, as well as on sleep, and melatonin and cortisol levels. A group of young, healthy participants was randomly allocated to either blue light blocking or control group. Depending on the group, participants wore amber contact lenses, reducing the transmittance of blue light by ∼90% or regular contact lenses for a period of 4 weeks. No changes were observed for measurements related to sleep and sleep–wake rhythm. Dim light melatonin onset, evening levels of melatonin, and morning cortisol answer did not show any significant alterations during blue light (BL) blockade. The significant effects were revealed both for sustained attention and visuospatial memory, i.e., the longer blocking the blue light lasted, the greater decrease in performance observed. Additionally, the follow-up session conducted ∼1 week after taking off the blue-blocking lenses revealed that in case of sustained attention, this detrimental effect of blocking BL is fully reversible. Our findings provide evidence that prolonged reduction of BL exposure directly affects human cognitive functioning regardless of circadian rhythmicity.
Highlights
Humans adapted their life to 24 h light–dark cycle
The comparison between groups was made in terms of age, gender, body mass index (BMI), sleep quality, level of daytime sleepiness, and chronotype for the group of 38 participants that completed the study [average age, 24.2 years (SD 3.9); 22 female; average BMI, 22.7 (SD 3.0)]
We investigated the effects of prolonged blocking of blue light on sustained attention and visuospatial working memory as well as on circadian rhythmicity
Summary
Humans adapted their life to 24 h light–dark cycle. As a diurnal species, we are exposed to light that is necessary for vision and constitutes a powerful modulator of non-visual functions. Human and animal studies provide evidence that the NIF system detects variations in ambient irradiance and elicits long-term modifications of circadian rhythms as well as acute changes in hormone secretion, heart rate, sleep propensity, alertness, core body temperature, retinal neurophysiology, pupillary constriction, and gene expression (reviewed by Vandewalle et al, 2009; Duda et al, 2020) In these unique ipRGC cells, the triggering signal transduction is accomplished by the photopigment melanopsin, which shows maximum sensitivity to the blue, i.e., short wavelength, part of the spectrum (∼480 nm; Berson et al, 2002; Hattar et al, 2002). Behavioral, biochemical, and neuroimaging studies, as well as subjective measurements, demonstrated that the sensitivity of the human circadian system and alerting and cognitive responses to light is blue-shifted relative to the three-cone visual photopic system, related to melanopsin phototransduction (see reviews by Cajochen, 2007; Vandewalle et al, 2009; Chellappa et al, 2011a)
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