A new study published in the journal Neurobiology of Sleep and Circadian Rhythms provides evidence that exposure to blue light can directly alter the baseline responsiveness of the human brain. The research suggests that while moderate levels of blue light may increase cortical excitability in young adults, higher intensities might diminish this effect. These findings indicate that the relationship between light and brain function is complex and appears to differ significantly between adolescents and young adults.
Light exerts a broad range of biological effects that are not related to vision, often referred to as non-image forming effects. These effects include the regulation of sleep, the synchronization of circadian rhythms, and the modulation of mood and alertness.
This process is primarily driven by a specific class of cells in the retina known as intrinsically photosensitive retinal ganglion cells. These cells contain a photopigment called melanopsin, which is maximally sensitive to blue wavelengths of light. When these cells detect blue light, they send signals to various parts of the brain that control alertness and cognition.
A fundamental aspect of brain function is cortical excitability. This term refers to the responsiveness of neurons in the cortex to incoming stimuli. Cortical excitability is central to cognitive function, as it dictates how the brain processes information and reacts to the environment.
Previous research has established that this excitability is not static. It fluctuates depending on how long a person has been awake and their circadian phase. Typically, excitability increases the longer a person stays awake. The relationship between excitability and performance is often thought to follow an inverted U-shape. This means there is an optimal level of excitability for peak performance, while too little or too much can lead to poorer cognitive outcomes.
Despite knowing that light affects alertness, it was not fully established whether light exposure directly alters cortical excitability. The authors of the current study aimed to bridge this gap in knowledge.
“Blue light is well known to influence circadian rhythms and alertness, but its effects on cortical excitability, a fundamental property of brain function that reflects how responsive neurons are to stimulation, are much less understood,” explained study authors Roya Sharifpour of the Université libre de Bruxelles and Gilles Vandewalle of the University of Liège.
“This is particularly relevant because most light-emitting diodes (LEDs) widely used in daily life, including in smartphones, tablets, computer screens, and indoor lighting, emit significant amounts of blue light. As a result, we all are exposed to blue light throughout the day and in the evening.”
“Adolescents represent an important population for studying the non-visual effects of light, as they are among the highest consumers of artificial light. Understanding these effects is crucial to provide optimal lighting recommendations that support both performance and health. Adolescence is characterized by major changes in sleep timing, ongoing brain development, and differences in eye physiology.”
“For example, teenagers typically have clearer lenses, larger pupils, and later chronotypes, a natural tendency to go to sleep and wake up later, which could increase their sensitivity to light,” the researchers explained. “Conversely, they may actually be less sensitive because they are often exposed to more outdoor or artificial light throughout the day, which can reduce their sensitivity to non-visual effects of light. These all suggest that adolescents may respond to light differently than adults.”
“Given the limited knowledge about how light affects cortical excitability and the potential differences between adolescents and adults, we aimed to investigate: (1) whether light influences cortical excitability in young adults, and (2) whether the response differs between adolescents and young adults.”
For their study, the researchers recruited twenty-eight healthy volunteers. The final sample included thirteen young adults between the ages of 19 and 30, and fifteen adolescents between the ages of 15 and 18.
The researchers screened all participants to ensure they had no history of sleep disorders, psychiatric conditions, or excessive use of caffeine and alcohol. They also excluded individuals with extreme “early bird” or “night owl” chronotypes. To control for the effects of sleep history, all participants maintained a strict, regular sleep-wake schedule for five days before the laboratory session. Compliance with this schedule was verified using wrist-worn activity trackers and sleep diaries.
On the day of the experiment, participants arrived at the laboratory in the early afternoon. This timing was chosen to accommodate the school schedules of the adolescent participants. Upon arrival, subjects underwent a period of adaptation in dim light to standardize their recent light history.
The core of the experiment involved exposing participants to three distinct light conditions using a tunable LED light box. The conditions included an orange light, which served as a control, a lower-intensity blue light, and a higher-intensity blue light. The orange light and the lower-intensity blue light were matched in visual brightness but differed in their activation of melanopsin.
The researchers measured cortical excitability using a technique called transcranial magnetic stimulation coupled with high-density electroencephalography (TMS-EEG). This non-invasive method involves placing a magnetic coil against the participant’s scalp. The coil delivers a brief magnetic pulse to a specific area of the brain, in this case, the superior frontal gyrus. This pulse induces a mild electrical current that activates neurons in that region. A cap containing 60 electrodes records the brain’s immediate electrical response to this stimulation. By analyzing the amplitude and slope of this response, the team could quantify how excitable the cortex was under each light condition.
During the brain stimulation sessions, participants simultaneously performed a visuomotor vigilance task. They used a trackball device to keep a moving cursor centered on a fixed target on a computer screen. This task required sustained attention and fine motor control. The researchers aimed to see if changes in light exposure would affect performance on this task and if performance correlated with the brain measurements. Each light session lasted approximately 12 minutes, with washout periods in dim light between sessions.
The researchers observed a distinct pattern in the young adult group. When exposed to the lower-intensity blue light, adults showed a significant increase in cortical excitability compared to the orange light condition. This confirms that light composition affects brain physiology, as the two lights appeared equally bright to the eye but had different biological effects.
However, this increase did not continue linearly. When the adults were exposed to the higher-intensity blue light, their cortical excitability did not increase further. Instead, the data showed a trend toward a decrease in excitability compared to the moderate blue light.
This pattern suggests an inverted U-shaped relationship in adults. Moderate blue light appears to stimulate the brain’s responsiveness, but excessive intensity may dampen this effect or result in diminishing returns. This mirrors the theoretical relationship between arousal and performance. The finding implies that more intense light is not necessarily better for brain function and that there is an optimal range for stimulating the cortex.
The findings for the adolescent group were notably different. Statistical analysis showed no significant changes in cortical excitability across the three light conditions for the teenagers. Their brain responsiveness remained stable whether they were exposed to orange light, moderate blue light, or high-intensity blue light. This lack of variation contrasts sharply with the modulation observed in the adult group.
Despite the differences in how light affected their physiology, both groups demonstrated a link between brain excitability and behavior. The researchers found a significant positive correlation between cortical excitability and performance on the vigilance task.
In both adolescents and adults, higher levels of excitability were associated with better performance on the tracking task. This relationship held true regardless of the specific light condition. It suggests that while light may or may not shift the baseline state of the brain depending on age, the state of the brain is a reliable predictor of how well a person can function.
The researchers also analyzed the spontaneous brain waves of participants while they rested with their eyes open. They specifically looked at theta and alpha waves, which are often used as markers of sleepiness and alertness. The study found no significant changes in these brain waves across the different light conditions for either age group. This indicates that the changes in excitability observed in adults were specific to the brain’s response to stimulation rather than a general shift in ongoing background brain activity.
“The key takeaway is that blue light can directly influence the brain’s baseline responsiveness,” the researchers told PsyPost. “Since cortical excitability underlies nearly all cognitive and behavioral functions, this effect is important. Our study suggests that during the day, blue light can increase cortical excitability, but intensity matters, more is not always better, and excessive exposure may actually reduce excitability.”
“Importantly, adolescents and young adults do not respond to light in the same way. This highlights that age and daily light habits can shape how the brain reacts to environmental stimuli, meaning that optimal light exposure may differ across age groups.”
But as with all research, there are some limitations. The sample size was relatively small, which can make it difficult to detect subtle effects. The experiment was conducted only during the afternoon. Biological responses to light can vary depending on the time of day and the individual’s circadian phase. Additionally, the highest intensity blue light session was always administered last to ensure study completion. This order could theoretically introduce fatigue effects, although statistical models attempted to account for this.
“Our study showed differential responses, but it remains unclear whether these differences are due to higher or lower sensitivity,” Sharifpour and Vandewalle explained. “Future research could include a wider range of light intensities and assess cortical excitability in darkness, comparing it to responses under blue light, to determine whether adolescents are more or less sensitive. Additionally, since our study was conducted during afternoon, future studies could investigate the effects of light exposure at different times of day on cortical excitability.”
“This work highlights how everyday environmental factors like light can influence fundamental neural processes. Adolescence is a dynamic developmental period, and understanding how light interacts with brain physiology may eventually help inform age-specific strategies to support daily functioning and well-being.”
The study, “Cortical excitability is affected by light exposure – Distinct effects in adolescents and young adults,” was authored by Roya Sharifpour, Fermin Balda, Ilenia Paparella, John Read, Zoé Leysens, Sara Letot, Islay Campbell, Elise Beckers, Fabienne Collette, Christophe Phillips, Mikhail Zubkov, and Gilles Vandewalle.
