Light allows us to see and has a huge impact on a number of physiological functions, including the regulation of our internal biological clock, hormones like melatonin, changes in body temperature, and even changes in alertness. For light to have these effects, it must first travel through the retinal ganglion cells (RGCs), which are the “output neurons” of the retina. For normal vision, light is detected by retinal photoreceptor cells (rods and cones), then the signal gets passed on to RGCs. For circadian rhythms, blue-wavelength is detected directly by RGCs and goes straight to the suprachiasmatic nucleus (SCN). So, information for both vision and maintaining circadian rhythms passes through the RGCs at some point, but for the latter, the light is received directly by the RGCs.
The spectrum of visible light can be distinguished according to its wavelength, with violet light being at the short end of the visible spectrum, and red light being at the longer end of the spectrum. A subset of RGCs are sensitive to short wavelength light (shown in Figure 1, short wavelength blue light is around 480 nm), which is particularly relevant to our internal body clock. It is this short wavelength light that is able to reduce melatonin levels in a dose-dependent manner, while longer wavelength light will have little to no effect on melatonin secretion. Melatonin is a hormone secreted by the pineal gland at night that helps synchronize peripheral clocks in the body to the SCN master clock. Exposure to blue light in the evening has been shown to improve reaction time and increase alertness. In addition, exposure to shorter wavelength light such as blue or green (but not the longer amber and red wavelengths) in the morning can cause the circadian clock to advance, as evidenced by an earlier evening onset of melatonin production.
The effects of blue light on cognition have begun to be explored in recent years. An interesting study conducted in an office building found that workers who were exposed to blue-enriched white light for four weeks reported increases in subjective alertness, performance, positive mood, and concentration in comparison to four weeks of white light exposure. Perhaps surprisingly, blue light can work about as well as caffeine for sustained performance on tasks requiring psychomotor functioning!
To learn how light affects the brain, functional magnetic resonance imaging (fMRI) studies have been used to show that blue light activates particular parts of the brain that can increase levels of norepinephrine (a hormone and neurotransmitter that serves to alert the brain and body) and influence brain regions involved in cognitive processing. One aspect of cognition is working memory, which refers to temporary storage and manipulation of information required to guide learning, comprehension, and decision-making. Some studies have looked at the effects of blue light exposure during working memory tests, but results have been equivocal. A recent review suggested that a blue light exposure of 30 minutes or longer may be needed to observe performance-enhancing effects, and so previous studies using shorter exposures may not have been sufficient to observe measurable changes.
Research in this area is fairly limited, and has yet to fully investigate whether exposure to blue light can alter cognitive performance after cessation of a single dose of daytime blue light. Previous studies have tested people during, but not after, light exposure. This study asked the question of whether the effect persists after the exposure by measuring working memory performance and associated brain activation after a prior 30-minute exposure of continuous blue-wavelength light.
Light exposure has a large impact on a number of physiological functions including hormonal cycles and cognitive performance, with different wavelengths (colors) having different impacts. The goal of this study was to measure working memory performance after participants were exposed to 30 minutes of blue light.