Let the sun shine in! (to your retinal ganglion cells)

Exposure to Blue Light Increases Subsequent Functional Activation of the Prefrontal Cortex During Performance of a Working Memory Task

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.

Who and what was studied?

Thirty-five healthy young adults (18-32 years old; 18 female, 17 male) underwent a period of controlled light exposure, followed by cognitive testing. Participants were asked to consume their normal amount of caffeine in the morning before coming into the lab. They were then put through a 30-minute blue-light washout period, sitting in a dark room while only being exposed to two amber lights, to reduce the effects of any outdoor or artificial light exposure they might have had in the morning.

The light treatment consisted of 30 minute exposures to either blue or amber light. The blue light was from the commercially available Philips goLITE BLU device that produces a narrow bandwidth of blue light. The amber lights came from the same manufacturer and fit into the same device. After the exposure period, participants underwent an fMRI scan, and then completed the working memory test approximately 40 minutes after completing the light exposure period.

The N-back test was used to measure working memory. This test presents a series of letters on a screen, one at a time. Three conditions were used; the control condition (“zero-back”) had participants identify whether or not the letter on the screen matched a predetermined letter, by pressing yes or no. In the “one-back” condition participants pressed a button to indicate whether the current letter presented was identical to the letter that was presented immediately before, and for the “two-back” condition participants indicated whether the letter shown on the screen was identical to the letter presented two letters previously. Each condition was tested for 42 seconds, and presented in random order for a total of nine blocks (3 “zero-back”, 3 “one-back”, and 3 “two-back”).

Thirty-five healthy male and female participants were exposed for 30 minutes to either blue or amber light, followed by functional magnetic resonance imaging (fMRI) and a test of working memory.

What were the findings?

Participants reported sleeping an average of 6.8 hours on the night before the assessment and consumed an average of 0.93 caffeinated products per day. Eight participants (four in each group) reported having one caffeinated product on the morning of their assessment. The main study findings are shown in Figure 2. Participants in the blue-light group responded faster than the amber light group during the one and two-back tests, while no differences were seen during the zero-back condition. To account for the tradeoff between speed and accuracy, a measure of cognitive throughput was calculated using both reaction time and accuracy of the responses. Again, there were no differences during the zero-back condition, but participants in the blue-light group showed significant improvement in the one-back condition and marginal improvement in the two-back condition. The accuracy scores were similar between groups, which means that participants exposed to blue light had faster response times with no decrease in accuracy.

Results of the fMRI showed that individuals in the blue-light group had increased brain activation in the dorsolateral and ventrolateral prefrontal cortex, areas that are known to be associated with working memory performance. Further analysis also showed a negative correlation between ventrolateral prefrontal cortex activation and response time, meaning greater brain activation led to faster response times. In addition, there were not any regions of the brain that were activated more in response to amber light than to blue light.

After exposure to blue or amber light, participants in the blue light group were faster in their responses to a test of working memory and showed increased brain activation of the dorsolateral and ventrolateral prefrontal cortex.

What does the study really tell us?

This study adds to the existing research by investigating the effects of sustained (30 minute) exposure to blue light and subsequent performance in cognitive testing while measuring areas of brain activation. The practical, everyday life applications of this data are somewhat limited due to the nature of the study design, but imply that getting some daylight exposure prior to working on mentally demanding tasks could be beneficial. The results suggest these effects may persist for at least 40 minutes after cessation of the blue light exposure, but the full time course of effects remains to be determined. This information could be useful when selecting office or hospital lighting, or any time someone needs to be mentally focused when they are forced to stay up overnight. The device used in the study is commercially available, and devices of this type could be a useful addition to an office setup that is deficient in natural light.

This study had several limitations worth noting. Participants were kept in a room without any blue light for 30 minutes, followed by one of the two light conditions (blue or amber) and then underwent the fMRI and cognitive testing. This means that people in the amber light group were not exposed to any blue light for over three hours, a situation that is usually only present during sleep. Most people would normally have some blue light exposure on their commute to work, as well as in their office, school, etc. This means that during normal day-to-day life, additional exposure to blue light could be less likely to have any measurable effects. Also, the laboratory experiments started at 7:45 a.m., which may have been too early in the day for some people to perform optimally, especially without normal light exposure prior to the cognitive testing.

This study contributes to the existing literature on light exposure and brain activation and shows benefits of blue-light exposure prior to cognitively demanding tasks. This information could be useful when choosing office or hospital lighting, or any time someone has to be mentally focused during sub-optimal sleeping conditions.

The big picture

This study suggests that light exposure can affect brain activation during working memory tasks, even after the light exposure has ended. In apparent contrast, other studies have shown that changes in brain response due to light exposure would decline within 10 minutes of termination. A likely reason for this discrepancy is that previous studies used light exposures lasting from less than one minute to 20 minutes, while the duration of light exposure in the current study was longer (30 minutes). Further research using varying combinations of light exposure and waiting periods is needed to determine how the effects of light persist on brain activation and cognitive performance testing.

Findings of the current study (correlating improved performance on the working memory test with increased activation of the ventrolateral prefrontal cortex) are also in line with previous research showing faster decision-making with similar brain activation. Other research has suggested that when the baseline activity in specific parts of the brainstem (the locus coeruleus, which is also involved in sleep/wake regulation) is higher, faster response times can be observed because a lower activation is needed to reach a threshold for response. As depicted in Figure 3, blue light can cause increased activation in that part of the brain, which then releases norepinephrine (also known as noradrenaline) and can affect working memory functions.

Further research is needed to determine the most effective patterns of blue light exposure for optimal cognition without compromising the body’s circadian rhythms and sleep pattern. Also, because the effects of blue light on performance may be affected by both genotype and time of day being tested, research on older individuals (for whom the effects of blue light on cognition may be reduced) is also needed.

Results from this study are in line with previous research and show that the effects of blue-light exposure may persist for at least 40 minutes after the end of exposure. This study also further confirms the connection between blue light exposure, activation of the prefrontal cortex, and improved working memory function.

Frequently asked questions

Can I use blue lights to compensate for getting too little sleep?

For a day or two maybe, but not chronically. The participants in this study were fully rested, and so it is unknown how the results would differ in a sleep-deprived state. However, there is some evidence that blue light can improve alertness and cognitive performance while driving overnight, and may be useful for extended road trips. That being said, please don’t test your sleepiness limits while driving with the help of blue light. Safety first.

Are some people affected more than others by blue light?

Possibly. The mechanisms of action will be the same, but the degree of brain activation could theoretically be different. This study did not address genetic variations of the participants, but variations in the melanopsin gene and the pupillary light reflex may cause different effects on brain activation by allowing variable amounts of light in. This genetic variation may account for part of why some people need to avoid blue light at night to sleep properly, while others can fall asleep with the lights on.

What should I know?

Exposure to just 30 minutes of blue light can significantly improve working memory performance in non-sleep-deprived people, with the effects persisting for at least 40 minutes after the exposure. When access to blue light is limited, it seems prudent to make an effort to get outside during the daytime prior to doing a task that requires your best mental performance, or use full-spectrum lighting. Further research is needed to establish the best protocols for light exposure, identify any differences between men and women and between younger and older people, and determine if there are specific genotypes that may be more affected.

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