Improving children’s alertness and neuromuscular response by using a blue-enriched white light in the kindergarten playroom

Abstract

Preschool children, who spend most of their time indoors, and the effects of artificial light on children’s health and performance are important. Previous studies show that blue-enriched white light (BWL) has significant effects on human bodies, but only a few studies have specifically examined its effects in young children. Moreover, due to the significant physiological differences between children and adults, findings from BWL studies in adults cannot be directly applied to children. Therefore, investigating the effects of BWL on young children living in indoor environments is crucial. We recruited 24 preschool children (age: 5 ± 0.8 years; 12 girls and 12 boys) to participate in a within-subject, randomized crossover study involving common white light (CWL) (450 lx, Melanopic EDI: 354.04 lx) and BWL (450 lx, Melanopic EDI: 746.05 lx) in a kindergarten playroom. Under different light conditions, the children underwent tests for cardiac activity and critical flicker fusion frequency (CFF), as well as psychomotor vigilance task (PVT) and ruler drop test (RDT). The results indicated that BWL had significant effects on preschool children. Compared to CWL exposure, BWL exposure significantly improved cardiac activity, alertness, and neuromuscular response but slightly increased visual fatigue. Our study reveals that BWL has significant potential to improve children’s physiological and cognitive functions, particularly to improve cardiac activity, alertness, and neuromuscular response. This study broadens the understanding of the effects of indoor lighting on children and provides a theoretical basis for designing a healthy indoor environment for children.

Introduction

Nowadays, people spend most of their time indoors and are exposed to indoor lighting throughout the day. The indoor lighting environment (both daytime and nighttime)^1,2 significantly impacts psychological and physiological health, affecting cognitive function³, work performance⁴, mood⁵, concentration^6,7, circadian rhythms^8,9, melatonin suppression^10,11, and sleep quality^12,13. Therefore, designing an indoor lighting environment tailored to human needs has become a pressing issue in both academia and society.

Since short-wavelength light (especially blue light) has been shown to significantly influence non-visual effects^14,15, such as circadian rhythms, researchers have incorporated blue light into standard white light to create blue-enriched white light (BWL), which has demonstrated significant effects on both human psychology and physiology¹⁶. A study has shown that exposure to BWL leads to greater improvements in cognitive flexibility in preschool children (aged 4.5–5.5 years)¹⁷. Research has shown that BWL exposure positively affected subjective alertness, mood, performance, night fatigue, attention, and eye discomfort in 94 adults (mean age: 36.4 years)¹⁸. BWL has also been reported to improve alertness, mood, and subjective visual comfort in 15 college students (mean age: 23.53 years) and to suppress melatonin levels¹⁹. Additionally, BWL has been found to increase cognitive processing speed and cognitive attention in 58 high school students (aged 16–20 years) and vocational school students (aged 18–29 years)²⁰. However, the effects of BWL vary with age. One study found that BWL exposure significantly affected alertness in 26 young participants (mean age: 25.0 years) and 12 older participants (mean age: 63.6 years), but changes in melatonin levels at night and cortisol levels are not consistent. The evening rise in melatonin was attenuated only in young participants, whereas older participants exhibited higher cortisol levels and reduced activity compared to the younger group²¹. They suggest that lighting conditions need to be adapted to the age of the targeted population, as human conditions vary across different ages, including pupil²², crystalline lens shape²³, brain structure²⁴, musculature²⁵, cardiac activity²⁶, cognitive²⁷, balance²⁸, muscle synergy²⁹, metabolic rate³⁰, and melatonin suppression³¹. Therefore, the effects of BWL exposure on the human body remain unclear and may vary by age, warranting further investigation and more comprehensive studies.

Preschool children are hypersensitive to the non-visual effects of light^32,33. Preschoolers spend most of their daily time in playrooms, which are their primary activity spaces in kindergartens. They play games, exercise, and study in these spaces, where lighting plays a crucial role in influencing physiology and behavior. Therefore, maintaining an appropriate lighting environment in playrooms is essential. The non-visual effects of light may influence not only cognitive function but also neuromuscular performance, support the learning and practice of exercise skills, and ultimately contribute to children’s cognitive and physical development.

However, there is limited research on preschool children and indoor light environments, but because of the physiological differences between preschool children and adults, existing findings for adults cannot be directly applied to preschool children. To date, studies involving the effects of the indoor light environment on preschool children are at an exploratory stage. Additionally, only a few studies have examined the effects of indoor BWL exposure on neuromuscular response in young children.

Therefore, this study aimed to determine the effects of indoor BWL on the psychology and physiology of preschool children. The experiment was conducted in the actual playroom of a kindergarten, a familiar environment for preschoolers, which enhances the relevance of the research findings. We evaluated changes in cardiac activity (measured using heart rate sensors), visual fatigue (measured using the critical flicker fusion frequency task [CFF]), alertness (measured using the psychomotor vigilance task [PVT]), and neuromuscular response (measured using the ruler drop test [RDT]) among preschool children exposed to common white light (CWL) and BWL. Our findings could provide a reference for establishing more scientific and modern indoor lighting standards for children and creating a healthy kindergarten playroom environment suited to children’s developmental needs.

Materials and methods

Participants

Since children aged 4 to 6 are the main participants in kindergartens, many kindergartens adopt mixed-age education, allowing these children to learn and live together. For this study, 24 healthy preschool children (this sample size meets the required number of participants for intervention studies targeting this age group, as supported by the references^34–36 were recruited from a kindergarten in Guangzhou, China. Before the study began, the parents or legal guardians of all participating children provided informed consent for their children’s participation. The study strictly followed the ethical principles outlined in the Declaration of Helsinki. Ethical approval for this research was obtained from the Ethics Committee of the Experimental Kindergarten under the Panyu District Education Bureau, Guangzhou (Approval Number: GGE-2023–0511 A). The inclusion criteria were as follows: (1) physically and mentally healthy, with no sports-related injuries within the last 6 months and no visible scars on the body; (2) no cross-country or cross-time zone travel within 3 months prior to the study; and (3) no use of psychotropic medication prior to the study and no ocular disorders (e.g., color blindness, color weakness, nearsightedness, and farsightedness).

The children’s health conditions were initially reported by their parents. Subsequently, the children underwent a physical examination to confirm their eligibility for the study. Besides, we confirmed with their parents that their sleep conditions before and during the study were normal. Therefore, all participants were healthy, and no sleep issues that could have affected the study. To rigorously investigate the effects of BWL on preschoolers, the participants’ sex and age were strictly controlled (Table 1).

Table 1.

The demographic information of the preschool children (Mean ± SD).

	Participants
Number of people	n = 24
Age (years)	5 ± 0.8
Gender Age & gender	Girls: 12, Boys: 12
4 years old	8 (4 Girls / 4 Boys)
5 years old	8 (4 Girls / 4 Boys)
6 years old	8 (4 Girls / 4 Boys)

Light environments and light intervention design

To prevent the experimental results from being affected by the children’s unfamiliarity with the environment, we conducted the experiment in an indoor playroom commonly used by kindergarten children. We used the AMBITFUL FL80RGB light (Ambitful Co., Ltd., Shenzhen, China) to create two indoor light environments: CWL (450 lx, Melanopic EDI: 354.04 lx), simulating the typical indoor light environment, and BWL (450 lx, Melanopic EDI: 746.05 lx) (Fig. 1). The spectroradiometer (HPS350J, HOPOOLight Detecting Instrument Co., Ltd., Hangzhou, China) was placed at the eye level (vertical plane) of children and directed toward the light source. Since the effects of BWL on preschool children are unclear, we adopted a relatively short-term exposure design based on the adults’ research protocol³⁷. The protocol lasted for 25 min including 10 min of CWL, 10 min of BWL and a 5-min washout period. During the light intervention, the playroom was kept quiet, with a constant temperature of 26 °C and humidity of 60%. Table 2 describes the detailed parameters of CWL and BWL. All 24 children participated in the experiment, and data collection occurred at the same time of day for each participant. Each child was observed and questioned at least three times during the experiment, and no psychological or physiological discomfort was reported under either light environment. Therefore, the experiment was conducted in a safe and controlled manner.

Fig. 1.

The spectrum of the common white light (CWL) and the blue-enriched white light (BWL).

Table 2.

The information of the light intervention.

	Common white light (CWL)	Blue-enriched white light (BWL)
Illuminance	450 lx	450 lx
Ra	93.6	88.3
Melanopic EDI	354.04 lx	746.05 lx
Room temperature	26 °C	26 °C
Room humidity	60%	60%
Duration	10 min	10 min

Protocol and measurements

To minimize the practice effect during the experiment, the preschoolers underwent a comprehensive full practice session lasting approximately 20 min before the formal testing to ensure that they could independently complete the test items. The playroom has windows, and only natural sunlight (without any artificial lighting) was present during the practice session and washout period. The light environment was controlled by the curtains at 50 lx, maintaining only the basic level of illumination during the practice session and washout period. Using a within-subject, randomized crossover study, the same set of tests was administered to all 24 preschool children, but with different sequences of light exposure. Specifically, one group of 12 children (6 girls and 6 boys) were first exposed to CWL, while the other group of 12 children (6 girls and 6 boys) were first exposed to BWL. All tests were conducted in different light environments of the experimental design. All children were seated during the light exposure (cardiac activity), CFF, and PVT tests. They rested prior to the measurement. Figure 2 outlines the specific testing procedure, and Table 3 details all the collected data from the tests.

Fig. 2.

The experimental process of study protocol.

Table 3.

The summary of experimental data in common white light (CWL) and blue-enriched white light (BWL) (Mean ± SEM).

Measurements	Common white light	Blue-enriched white light	p value	Effect size
The cardiac activity
HR (bpm)	98.08 ± 2.17	96.42 ± 1.94	< 0.05*	0.431
Min HR (bpm)	80.54 ± 2.26	77.54 ± 1.85	< 0.05*	0.524
Max HR (bpm)	114.17 ± 1.86	114 ± 2.21	0.899	0.026
HRV (ms)	56.04 ± 1.74	57.50 ± 1.37	0.111	0.338
RMSSD (ms)	43.91 ± 4.78	45.45 ± 3.39	0.588	0.112
SDNN (ms)	51.22 ± 3.21	54.47 ± 2.94	0.204	0.267
PNN50 (%)	20.83 ± 3.61	24.71 ± 3.21	< 0.05*	0.460
LF power (ms2)	873.82 ± 110.71	1136.12 ± 198.31	0.263	0.234
HF power (ms2)	849.76 ± 192.47	929.21 ± 149.63	0.712	0.076
LF/HF ratio	2.63 ± 0.75	1.62 ± 0.21	0.194	0.273
The critical flicker fusion frequency
Mean threshold (Hz)	34.88 ± 0.61	33.83 ± 0.7	< 0.01**	0.644
Forward threshold (Hz)	34.44 ± 0.73	33.18 ± 0.86	< 0.01**	0.593
Backward threshold (Hz)	35.32 ± 0.57	34.48 ± 0.72	< 0.05*	0.448
The psychomotor vigilance task
Mean reaction time (ms)	1189.71 ± 53.26	1142.50 ± 52.94	< 0.05*	0.458
Median reaction time (ms)	1118.50 ± 47.36	1059.83 ± 51.6	< 0.05*	0.499
Slowest 10% reaction time (ms)	1883.17 ± 118.23	1840.40 ± 103.99	0.570	0.118
Fastest 10% reaction time (ms)	883.46 ± 38.1	848.69 ± 37.51	< 0.05*	0.494
The ruler drop test
Mean reaction length (cm)	55.50 ± 3.29	44.84 ± 3.51	< 0.01**	0.829
Dominant hand
Mean reaction length (cm)	56.43 ± 4.03	45.12 ± 3.62	< 0.01**	0.693
Min reaction length (cm)	36.08 ± 2.69	28.56 ± 2.54	< 0.01**	0.620
Non-dominant hand
Mean reaction length (cm)	54.58 ± 3.56	44.56 ± 4.43	< 0.05*	0.554
Min reaction length (cm)	38.81 ± 2.15	28.04 ± 2.66	< 0.01**	0.977

HR, heart rate; Min, minimum; Max, maximum; HRV, heart rate variability; SDNN, standard deviation of normal to normal intervals; RMSSD, square root of the mean normal to normal interval; PNN50, percentage of adjacent pairs of normal to normal intervals differing by more than 50 ms; LF, low frequency; HF, high frequency; bpm, beats per minute; Hz, hertz; ms, millisecond; cm, centimeter.

Significant values are in [bold].

Cardiac activity

The preschool children’s cardiac activity was monitored using the Polar H10 Heart Rate sensor (Polar Electro, Finland). Prior to the experiment, each child wore a Polar H10 chest strap connected via Bluetooth to the Elite HRV application (version 5.5.4). Upon exposure to CWL or BWL, cardiac activity was recorded for 1 min using the Elite HRV App, known for its capability to swiftly capture and calculate cardiac activity indicators³⁸. HRV data indicate both sympathetic and parasympathetic nervous system activities³⁹ and can be used to evaluate stress, recovery, or rest status^40,41. In this study, HRV was measured using time-domain and frequency-domain indicators such as the standard deviation of normal-to-normal interval (SDNN), square root of the mean normal-to-normal interval (RMSSD), percentage of adjacent pairs of normal-to-normal intervals differing by more than 50 milliseconds in the recording (PNN50), low frequency (LF) power, high frequency (HF) power, and LF/HF ratio⁴².

Visual fatigue

Visual fatigue among preschool children was assessed using the critical flicker fusion frequency (CFF) test. CFF refers to the frequency at which flickering light is perceived as continuous light after it reaches a certain frequency⁴³. Reflecting the maximum speed of flickering light perceived by the visual system^44,45, CFF is an important indicator of visual fatigue⁴⁶. In this study, the visual fatigue test was conducted immediately after light exposure, and we defined it as visual fatigue resulting from 1 min of light exposure. CFF was measured using the BD–II–118 light-spot flash fusion frequency meter (Beijing Qingniaotianqiao Instrument & Equipment Co., Ltd, China). This device has a flickering light stimulus frequency that can be adjusted within a range of 10–60 Hz. Two types of CFF were measured: (1) forward threshold, in which the flicker frequency was adjusted from 10 to 60 Hz, and (2) backward threshold, in which the flicker frequency was adjusted from 60 to 10 Hz. In this study, mean frequency (the average of the forward and black thresholds) was used to measure visual fatigue. A lower mean frequency indicates a decrease in the frequency distinguished by human eyes, indicating increased visual fatigue. Conversely, a higher mean frequency suggests an increase in the frequency distinguished by human eyes, indicating decreased visual fatigue.

Alertness

The preschoolers’ alertness was measured using the psychomotor vigilance task (PVT), which is a widely accepted method for measuring behavioral alertness⁴⁷ and is commonly used as the standard for assessing alertness in research settings⁴⁸. In this study, the PVT was conducted using PC-PVT v2.0⁴⁹, with all measurements recorded through this software. The PVT test for 3 min under CWL or BWL. Given that preschool children may not be familiar with using a mouse, a button was used instead. Prior to starting the PVT test, researchers determined the preschool children were all right-handed, and the children placed the dominant hand in a fixed position beside the button. During the test, a visual stimulus appeared randomly on the screen every 2 to 10 s. Participants were instructed to quickly press the button upon seeing the stimulus and then promptly return their hand to its original position beside the button. All participants understood and successfully completed the PVT test independently before it officially began. The mean reaction time (ms) serves as an indicator of alertness and performance in this study. Specifically, a decrease in the mean reaction time suggests improved alertness, indicating better performance⁵⁰. Conversely, an increase in the mean reaction time suggests reduced alertness, indicating poorer performance. Other indicators of alertness included the slowest and fastest 10% reaction time⁴⁷.

Neuromuscular response

Neuromuscular response in preschoolers was measured using the ruler drop test (RDT)⁵¹. The RDT is a simple response task that is usually used to study human response in sports performance^52–54 and has been validated for use in preschool children⁵⁵. During the RDT, each child extended their index finger and thumb while a researcher held a ruler vertically, aligning the zero mark with the child’s fingers. Once ready, the researcher released the ruler at random intervals (typically 1 to 5 s). The distance from the child’s thumb to the zero mark (reaction length) was measured. To account for differences in hand dominance, the RDT was performed six times—three times using the dominant hand and three times using the non-dominant hand. The mean reaction length (cm) is an indicator of neuromuscular response. A decrease in mean reaction length indicates better neuromuscular response⁵⁶. Overall, the indicators of neuromuscular response included mean reaction length (i.e., average length of reaction with the use of the dominant and non-dominant hands), dominant and non-dominant hand mean reaction length (i.e., average length of reaction using the dominant and non-dominant hands), and minimum reaction length of dominant and non-dominant hand.

Statistical analysis

Statistical analyses of the test results were performed using the GraphPad Prism (version 9.5.0, GraphPad Software, USA). The Shapiro–Wilk test was conducted to assess the normality of the data distribution. The data conformed to normal distribution; thus, the paired-samples t-test was conducted to assess changes in participants under the two light environments, and p < 0.05 indicated statistical significance. Cohen’s d effect sizes were shown as follows: small (0.2 ≤ Cohen’s d < 0.5), medium (0.5 ≤ Cohen’s d < 0.8), and large (Cohen’s d ≥ 0.8).

Results

Our study results demonstrated significant effects of BWL exposure on preschool children (Table 3). Analysis of the Polar H10 Heart Rate sensor data revealed significant differences in cardiac activity between CWL and BWL exposures (Fig. 3). Specifically, under CWL exposure, their average heart rate (HR) was 98.08 bpm, whereas, under BWL exposure, it reduced to 96.42 bpm (p < 0.05; Cohen’s d = 0.431). Similarly, their minimum heart rate (Min HR) was 80.54 bpm with CWL and 77.54 bpm with BWL (p < 0.05; Cohen’s d = 0.524), indicating a decrease under BWL exposure. Moreover, the percentage of adjacent normal-to-normal intervals differing by more than 50 ms (PNN50) was 20.83% under CWL and 24.71% under BWL (p < 0.05; Cohen’s d = 0.460), showing an increase with BWL exposure. No other measurements were found to be significantly different between the two light environments.

Fig. 3.

The results of cardiac activity for the common white light (CWL) and blue-enriched white light (BWL) (mean ± SEM). The preschool children had significantly better cardiac activity in the BWL compared to the CWL. *: p < 0.05.

The measurement results of the CFF test revealed significant differences in preschool children’s visual fatigue between CWL exposure and BWL exposure (Fig. 4). A higher threshold indicates less visual fatigue. Their mean CFF threshold was 34.88 Hz under CWL exposure and 33.83 Hz under BWL exposure. Compared to CWL exposure, BWL exposure significantly reduced their mean threshold (p < 0.01; Cohen’s d = 0.644). In the forward threshold test, preschoolers exhibited 34.44 Hz under CWL exposure and 33.18 Hz under BWL exposure. BWL exposure significantly lowered their forward threshold compared to CWL exposure (p < 0.01; Cohen’s d = 0.593). During the backward threshold test, they demonstrated 35.32 Hz under CWL exposure and 34.48 Hz under BWL exposure. BWL exposure also reduced their backward threshold compared to CWL exposure (p < 0.05; Cohen’s d = 0.448).

Fig. 4.

The results of critical flicker fusion frequency (CFF) for the common white light (CWL) and blue-enriched white light (BWL) (mean ± SEM). The preschool children had less visual fatigue in the CWL compared to the BWL. *: P < 0.05; **: P < 0.01.

The results from the PVT indicated significant differences in preschool children’s alertness between CWL exposure and BWL exposure (Fig. 5). Their mean reaction time was 1,189.71 ms under CWL exposure and 1,142.50 ms under BWL exposure. BWL exposure reduced their mean reaction time compared to CWL exposure (p < 0.05; Cohen’s d = 0.458). Their median reaction time was 1,118.50 ms under CWL exposure and 1,059.83 ms under BWL exposure. Similarly, BWL exposure reduced their median reaction time compared to CWL exposure (p < 0.05; Cohen’s d = 0.499). In terms of the fastest 10% reaction time, they exhibited 883.46 ms under CWL exposure and 848.69 ms under BWL exposure. BWL exposure also reduced their fastest 10% reaction time compared to CWL exposure (p < 0.05; Cohen’s d = 0.494). No other results showed significant differences between the two light environments.

Fig. 5.

The results of the psychomotor vigilance task (PVT) for the common white light (CWL) and blue-enriched white light (BWL) (mean ± SEM). The preschool children were significantly more alert in the BWL compared to the CWL. *: p < 0.05.

The results from the RDT indicated significant differences in preschool children’s neuromuscular response between CWL exposure and BWL exposure (Fig. 6). A shorter reaction length indicates a better neuromuscular response. Their mean reaction length was 55.50 cm under CWL exposure and 44.84 cm under BWL exposure. BWL exposure significantly reduced their mean reaction length compared to CWL exposure (p < 0.01; Cohen’s d = 0.829).

Fig. 6.

The results of the ruler drop test (RDT) for the common white light (CWL) and blue-enriched white light (BWL) (mean ± SEM). The preschool children had significantly better neuromuscular response in the BWL compared to the CWL. *: p < 0.05; **: p < 0.01.

When using the dominant hand, their mean reaction length was 56.43 cm under CWL exposure and 45.12 cm under BWL exposure. Their minimum reaction length was 36.08 cm under CWL exposure and 28.56 cm under BWL exposure. Compared with CWL exposure, BWL exposure reduced both the mean (p < 0.01; Cohen’s d = 0.693) and minimum (p < 0.01; Cohen’s d = 0.620) reaction lengths.

When using the non-dominant hand, their mean reaction length was 54.58 cm under CWL exposure and 44.56 cm under BWL exposure. The minimum reaction length was 38.81 cm and 28.04 cm when they were exposed to CWL and BWL exposures, respectively. Compared with CWL exposure, BWL exposure reduced both the mean (p < 0.05; Cohen’s d = 0.554) and minimum (p < 0.01; Cohen’s d = 0.977) reaction lengths.

Discussions

This study examined the effects of exposure to BWL on preschoolers in the kindergarten playroom. To precisely investigate how light affects the physiology of preschool children, we conducted our tests in a controlled environment with stable temperature and humidity. The results showed that BWL exposure improved their cardiac activity, alertness, and neuromuscular response but increased their visual fatigue.

Previous studies have highlighted the significant impact of light environments on human autonomic nervous activity⁵⁷. Blue light induces both sympathetic and parasympathetic nervous activity, leading to changes in heart rate. Some previous studies on adults reported that exposure to blue light increased adults’ heart rate (sympathetic activity)^58–60. However, another study involving 20 college students showed that exposure to blue light increased parasympathetic activity⁴². The differences between these findings might be due to methodological differences (such as the timing of light exposure, light intensity, duration of exposure, and type of light source). Our observations revealed that preschool children exposed to BWL exhibited notable changes in cardiac activity, specifically a decrease in heart rate and an increase in PNN50 (parasympathetic activity). This reduction in heart rate is beneficial for reducing cardiac workload⁶¹, potentially lowering the risk of cardiovascular disease morbidity and mortality⁶². In our study, BWL exposure also increased the preschool children’s PNN50 compared to CWL exposure, indicating heightened parasympathetic activity and reduced stress⁶³. Together, these results suggest that indoor BWL may positively influence heart health in children.

This study investigated the impact of BWL on preschool children’s visual fatigue. Our findings revealed that BWL exposure increased visual fatigue compared to CWL exposure, suggesting that BWL may exacerbate visual fatigue in children. This observation aligns with a previous research finding indicating that BWL tends to amplify visual fatigue more than other lighting environments⁶⁴. Although the light exposure in this study was shorter than in previous studies, with a duration of only one minute and no visual task involved, BWL still increased visual fatigue in preschool children. This could be because preschool children are sensitive to light, and blue light with higher energy affects them quickly. Besides, visual fatigue can stem from various factors beyond BWL environments. It can arise from differences in light color^65,66, illuminance levels⁶⁷, or correlated color temperatures⁶⁸. Furthermore, in previous studies, the CFF was approximately 28 Hz in 12 healthy participants (mean age: 22–26 years) in both BWL and non-BWL environments⁶⁴. Another study reported a CFF of 33.27 Hz in a common white LED light environment and 32.99 Hz in a modified high-illumination environment for 14 college students (mean age: 21.9 years)⁶⁹. In our study, the CFF was 34.88 Hz in a CWL environment and 33.83 Hz in a BWL environment, indicating a reduction of 1 Hz with BWL exposure. Therefore, although BWL did increase visual fatigue in children, the impact remained within a relatively safe range. However, it is important to remain vigilant about the effects of BWL on preschool children’s visual fatigue.

Light can have a variety of effects on the human body, with light intensity playing a significant role. Higher light intensity is known to enhance alertness, cognition, mood, and physiological arousal⁷⁰. To accurately assess the impact of light on alertness⁷¹, we maintained an illuminance of 450 lx in each light environment to eliminate variability in light intensity. This rigorous approach enabled us to demonstrate that BWL significantly influenced the alertness of 24 preschool children (aged 4–6 years). Compared to CWL exposure, BWL exposure reduced their reaction time from 1,189 ms to 1,142 ms (a 4% improvement in alertness), indicating a positive effect of BWL on children’s alertness. This finding is consistent with previous research findings indicating that BWL environments have positive effects on alertness⁷². The Intrinsically Photosensitive Retinal Ganglion Cells (ipRGCs) in our eyes play a crucial role in the non-visual effects of light⁷³. Blue light exposure affects ipRGCs, which contain melanopsin, a photopigment with peak absorption around 480 nm, making exposure to BWL more likely to induce non-visual effects^74–78. This may explain why BWL illumination increased the preschoolers’ alertness. In addition, the reaction time of preschool children was much slower than that of adults. This could be because children of this young age are easily distracted during the PVT test, and their neural and muscular development is still immature. We look forward to more PVT test results in preschool children in the future.

Our study found that BWL exposure had a positive impact on the neuromuscular response of preschool children. To mitigate the influence of dominant and non-dominant hands, we conducted separate tests when children used each hand. The results consistently demonstrated significant improvements in neuromuscular response with BWL exposure. Compared to CWL exposure, BWL exposure reduced the mean reaction length by approximately 10 cm, regardless of whether the dominant or non-dominant hand was used. Additionally, it reduced the minimum reaction length by approximately 8 cm when the dominant hand was used, and by 10 cm when the non-dominant hand was used. This indicated that BWL exposure led to a more pronounced reduction in minimum reaction length when children used their non-dominant hands. To date, little is known about the effects of light on children’s neuromuscular response. One previous study indicates that high CCT light (BWL) improves elite swimmers’ performance, including reaction time and swimming speed⁷⁹. Our findings align with this trend. We hypothesize that the improvement in neuromuscular function results from the non-visual effects of BWL.

Although this study conducted a within-subject, randomized crossover study to investigate the effects of different light environments in the kindergarten playroom on children, it was not without limitations. First, we did not use the more accurate instruments of electroencephalography and electromyography to examine the participants because of the limitations of the experimental conditions in the real kindergarten playroom. Second, we considered introducing subjective mood scales in the study, but ultimately rejected them because we were not sure of their applicability to the specific conditions of light environment and preschool children. To comprehensively investigate the effects of BWL on preschool children in future studies, it will be essential to include the appropriate subjective tests such as mood and fatigue scales for children, along with advanced measurement modalities such as electroencephalography and electromyography, which will provide deeper insights.

This was a novel study. To our knowledge, the tests used in the experiment (except for the RDT) were validated for the first time in 3-6-year-old preschool children in a kindergarten. The limited current research on light exposure in preschool children has constrained our choice and application of measurement tools. As a result, some of our findings can only be compared with data from adults. Although this comparison may be a limitation, it provides a valuable starting point for future research in this field. We encourage more researchers to pay attention to preschool children’s health and study the relationship between the light environment and children’s performance.

Finally, this study indicates the importance of indoor lighting on children’s health and underscores the significant role of BWL in enhancing both physical and cognitive functions in children, providing a reference for the light environment design in the kindergarten playroom.

Conclusions

This study investigated the impact of BWL exposure on preschool children, comparing it with CWL exposure. Our findings indicate several significant benefits of BWL. Specifically, BWL exposure enhances children’s cardiac activity (PNN50), reducing both cardiac load and stress. Furthermore, exposure to BWL improves alertness, thereby possibly enhancing concentration in learning and daily activities. In the neuromuscular response experiment, children performed best under BWL exposure, potentially enhancing their neuromuscular response. However, it is important to note that BWL use was associated with slightly increased visual fatigue during practical application. To optimize these benefits while mitigating potential drawbacks such as visual fatigue, future applications of BWL should include thoughtful lighting design and time management strategies. Our findings provide a reference for integrating BWL into children’s daily environment.

Author contributions

Yankang Jiang conducted subject recruitment, the intervention experiment, data analysis, and the writing of the original manuscript. Xiaodong Hu contributed to the review and editing. Peijun Wen led the project, contributing to the conceptualization, study design, project administration, and supervision of the writing. All authors discussed the results and prepared the final manuscript.

Data availability

The datasets analyzed during the current study are available from the corresponding author on reasonable request.

Declarations

Competing interests

The authors declare no competing interests.