Introduction
Biological processes such as cardiac, endocrine, and brain processes have
standardized rhythms under the environmental stimulation of the presence (or
absence) of light [1]. Vision is a means of
receiving light stimuli and maintaining circadian synchronism, as one of the most
important senses in humans. Light exposure is the main stimuli for synchronizing
circadian rhythms, releasing hormonal alterations, and influencing motor and
cognitive tasks and even athletic performance [1].
In this context, melatonin plays a role in the regulation of a variety of
physiological processes such as the circadian clock in the suprachiasmatic nucleus,
vascular response, reproduction, sleep and cognition [2]
[3].
Light exposure to different illumination patterns is a feasible and low-cost tool
and could help improve depression and anxiety recovery, alertness state, and
performance. The relationship between exposure to light and decreased melatonin
secretion has been documented [4], and sports
scientists showed that lower levels of melatonin were associated with greater speed
in reaction time, an important indicator of the cognitive processes in sports [5]
[6]
[7]
[8]
[9]
[10].
Volleyball is a team sport involving intermittent and unpredictable actions, with
intense physical demands interspersed with moments of pause [11]. Volleyball players must have a high
cognitive and attention flexibility (broad and selective) for excellent performance
[12]. However, competitions often take
place at night, when most athletes are past their peak performance period of the
day, and exposure to light can lead to a decrease in melatonin levels. A lower
melatonin level might decrease the natural loss in reaction time that occurs after
long periods of wakefulness [13].
The effect of monochromatic blue can be even greater in suppressing melatonin levels
[14]
[15]. However, there is scarce knowledge about the effects of wearing
glasses with blue lenses on improving sports performance and pre-activation of the
motor cortex of the central nervous system. Any improvement in alertness, and
cognitive and physical performance potentially could benefit sports performance
(e. g., volleyball) and have a practical application. Thus, the aim of the
present study was to analyze whether wearing glasses with blue lenses under an
artificial light pattern would influence the melatonin level, physical and cognitive
performance of youth volleyball players. Our hypothesis was that wearing blue lens
glasses would promote acute suppression of melatonin production (via modulation of
the central nervous system), triggering improvements in alertness, cognitive
performance, and agility.
Materials and Methods
Subjects
Fifteen youth volleyball players participated in this study (15.1±1.5
years; 180.9±11.5 cm; 76.6±13.9 kg, body mass
index 23.4±3.9 kg/m2). Their usual awakening
time was between 6:30 am and 10:40 am. The research project was approved by the
local Ethics and Research Committee (n. 68569417.5.0000.5147). The volunteers
and their parents signed a free and informed consent form before the beginning
of the experiments. The following inclusion criteria were used: 1) male gender,
2) participant in competitive youth volleyball team, 3) abstained from exercise
within 24 hours prior to testing, and 4) having a chronotype between 1
(moderately morning) and 5 (moderately evening). Since our goal was to avoid a
predominantly morning or evening sample, we believed that the above chronotype
description was an inclusion criterion.
Exclusion criteria were: 1) having a history of injuries that compromised the
tests; 2) having used dietary supplements or medications that could affect
performance for at least 2 weeks before testing; 3) present health problems that
prevented the procedures from being performed; 4) having any visual impairment
related to the distinction and visualization of colors; and 5) having changed
time zones in the week before the tests.
Experimental design
The players attended the laboratory on 3 occasions with a 48-h interval in
between. On the first visit, they were characterized by age, body mass, height,
and body mass index (weight/height2), and reported their time
of experience in volleyball. The athletes also received the Pittsburgh Sleep
Quality Index (PSQI) [16] and the
Morningness-Eveningness Questionnaire [17]
to be delivered completed on test days. They were then asked to abstain from
intense physical activity and the use of alcoholic beverages and/or
stimulants 24 hours prior to the tests, and to maintain their usual
bedtime and waking time the day before the tests and on test days. Finally, the
athletes were familiarized with the procedures of the experiment.
On the other two visits, the glasses (blue or transparent) were randomly selected
for the participants, who performed the experimental procedures in a
counterbalanced manner. Initially, the athletes were sent to the laboratory
where they remained seated with their arms and head supported, avoiding any kind
of sudden movement. This position was maintained for a period of
10 minutes in “total darkness” with their eyes totally
blindfolded by a mask and with all the lamps out, with 0 lux, verified through
the lux meter (Victor 1010 A Auto Digital Lux Meter). The period of
"total darkness" was followed by the first collection of saliva,
which was labeled and stored in an ultra-freezer at –80ºC. Soon
after, the individuals were submitted to light stimulation. The light stimulus
was modified by wearing the blue or transparent lenses (according to a drawing).
The use of lenses was carried out only during the period in which the subjects
remained in the condition of light stimulation. At the end of this period, each
participant was asked, “How do you feel right now?”. The
athletes also checked the Karolinska Sleepiness Scale [7]
[18]
[19], their current state of
alertness or drowsiness, and the Visual Analogue Mood Scale [7, 20]. Upon completing these procedures, a
salivary melatonin sample was again collected, labeled, and stored in an
ultra-freezer. After the second collection of the saliva sample, the athletes
were taken to an annex of the physiology laboratory for the Attentional Network
Test [21]. Thereafter, they were taken a
little further to the yard for the T-test. [Fig. 1] shows the experimental design of the study.
Fig. 1 Experimental Design of the Study.
All data relating to the attentional network test and the T-test were noted on a
form and allocated to a file within the physiology laboratory in conjunction
with the Sleep Quality Morningness-Eveningness Questionnaire, Visual Analogue
Mood Scale, Karolinska Sleepiness Scale, and subjective recovery scale.
The environment
The ambient lighting was provided by 12 fluorescent lamps of the Philips TLD
32 W/840-NG Super 84 Eco Master tube, each with a luminous flux (lumens
- lm) of 2700 lumens, a temperature of 4,100 kelvin, a wavelength of
approximately 550 nanometers, a power of 32 watts, with a total size of 121
centimeters. They were positioned in pairs horizontally by means of six TBS050
luminaires (Embed), with a general diffuse lighting characteristic, and a
distance of 2.50 meters from the light source to the ground. The average lux of
the laboratory was calculated using the formula: Luminous flux
(lumens)/illuminated surface area (meters2), calculated as
(2,700×12)/39.4 ≅ 822 fluxes, complying with ICS 91.160.10. The
temperature (22±1.0 ̊ C) and relative humidity
(66±2.0%) of the environment were controlled.
Blue monochrome color lenses
The lenses ([Fig. 2]) are composed of
yellow, green, and red spectrum protection lenses (Blue Safety Glasses 492
nm–770 nm) in a blue monochrome color and transparent lenses
(without protection). [Fig. 2] shows the
blue lens goggles used.
Fig. 2 Blue Safety Glasses.
Morningness-Eveningness Questionnaire
To identify the chronotype, the Horne and Ostberg's
Morningness-Eveningness Questionnaire [17]
was used, translated into Portuguese by Benedito-Silva et al. [22]. This is a questionnaire of
self-assessment that categorizes a person based on their preference for
performing routine activities in the morning or evening. The result is a
numerical value that varies between 16 and 86 points, classifying the individual
in 5 (five) different types: extreme afternoon (16 to 30 points), moderately
afternoon (31 to 41 points), indifferent (42 to 58 points), moderately morning
(59 to 69 points) and extreme morning (70 to 86 points).
Sleep Quality Index - PSQI
The PSQI [16] translated into Portuguese
[23] was applied to assess the
subjects’ sleep quality for the last month. The questionnaire consists
of 19 self-administered questions and 5 questions answered by a roommate (only
used for clinical evaluations). Thus, the 19 questions are grouped into 7
components, distributed on a scale of 0 to 3. These PSQI components are divided
into subjective “sleep quality,” “sleep
latency,” “sleep duration,” “habitual sleep
efficiency,” “sleep disorders,” “use of sedative
medications,” and “daytime dysfunction.” Therefore, the
scores for these components are added to produce a global score, which ranges
from 0 to 21, with the higher the score, the worse the quality of sleep. A PSQI
greater than five indicates that the individual is experiencing great
difficulties in at least 2 components or moderate difficulties in more than 3
components.
Illuminance measurement
Illuminance is defined as the luminous flux, the amount of light that reaches a
certain point. The unit of measure of illuminance is expressed in lux and was
measured using the Victor 1010 A Auto Digital Lux Meter. According to
ICS 91.160.10, to correctly use the lux meter, the evaluator must maintain the
device at an illuminance similar to that of the environment for 5 to
10 minutes for stabilization, perform measurements on the work plane,
and maintain a minimum distance of 2 meters from the lux meter cell so that the
luminous flux is not influenced by the person taking the measurement. All
measurements were performed at the participant's eye level.
Salivary melatonin measurements
All pre-test samples were collected between 18:00h–18:30 h, the
period in which the beginning of the melatonin synthesis threshold in low light
begins to increase significantly. All samples were collected in a tube (salivary
kit collection 1 ml - melatonin), with all individuals seated, with at
least 0.5 milliliters of saliva collected. The melatonin analysis was performed
using the Automated Enzyme Immunoassay analysis kit (IBL International, Hamburg,
Germany). The minimum detectable dosage of melatonin (analytical sensitivity)
was determined to be 0.30 pg/ml. Salivary samples were collected
following previous recommendations [24]
[25].
The participant stimulated the production of saliva and deposited it in the
bottle until reaching the equivalent of at least 0.5 ml. Immediately
after salivary collection, the vial was identified with the athlete’s
name and labeled pre or post and blue or transparent lenses. Duly identified, it
was deposited in a polystyrene box in an upright position and stored in an
ultra-freezer at a temperature of –80º C for further
analysis.
Karolinska Sleepiness Scale
In order to check the subject’s state of alert and sleepiness, the
Karolinska Sleepiness Scale [18] was
applied. The scale consists of 9 points, where each item features a
characteristic: 1=very alert, 3=alert, 5=neither alert
nor sleepy, 7=sleepy, 9=very sleepy. After the light exposure,
the subjects were asked to visualize the scale to verify their real state at the
moment.
Visual Analogue Mood Scale
The Visual Analogue Mood Scale [26],
translated to Portuguese [20], consists of
16 items. Each of them was represented by a straight line of 100 millimeters
connecting two opposite feelings. Four intuitive factors were combined into
these items: anxiety, physical sedation, mental sedation, and other feelings.
Before applying the scale, previous training was carried out, featuring oral
instructions and practical examples about the scale. Furthermore, it is
important that oral instructions emphasize that both ends of the line should be
considered the maximum the subject can feel with respect to that item and the
center is equivalent to its usual state. Hence, the subject filled each item by
crossing the line that links the two opposite characteristics in all sixteen
items on the scale.
Attentional Network Test
The Attentional Network Test (ANT) developed by Fan et al. [21] includes a computer test featuring
“opposed” tasks and their respective answers verified through
the reaction time (RT) in milliseconds and percentage of correct answers. In
addition, the ANT requires the subject to determine in a set of five arrows
whether the central object is pointing left or right. As a reference, the arrows
appear above or below central point in the screen, accompanied or not by arrows
indicating opposite (incongruent) or equal (congruent) sides. Thus,
ANT’s efficiency is assessed by measuring how the RT is influenced
through the warning tips, spatial tips, and congruence of the arrows. Moreover,
executive functions such as alertness and guidance are also assessed through the
ANT. The session consists of a practical part that lasts for 2 minutes
and contains a block of 24 models, presenting the right and wrong answers during
the subject’s practice. After that, the experimental model is generated,
showing a total of 3 blocks of 5 minutes, each block presents 96 models
randomly, without any feedback response. Therefore, the ANT test is used to test
refined motor skills, with a high demand for precise movement made by small
muscle groups, which generally involves high levels of coordination between the
eyes and hands [13].
T-test
An adapted version of the T-test [27] was
used with its measurements reduced. This version of the test consists of a
frontal and posterior move of 5 (five meters), and two opposed lateral moves of
2.5 (two meters and fifty centimeters).
The test site was previously marked with orange paint spray for positioning the
“T” shaped cones. Four cones 24 cm high were placed in
each marked space and named A, B, C and D. The athlete had to stay just behind
cone A and wait for the call to start that was given from the countdown of 3
(three) seconds. The athlete quickly moved forward to cone B, then moved
laterally to cone C and laterally to cone D. Finally, the athlete moved
laterally to cone B and later to the beginning at cone A. When the subject
reached the cones B, C, and D, a squat movement was performed, followed by
touching the fingertips to the respective cones. The total test time (in
seconds) was recorded by two evaluators using two Samsung Galaxy S6 stopwatches
when the athlete reached cone A, signaling the end of the test. Trials with more
than 3% difference were not considered.
Subjective Recovery Perception Scale
To verify the athletes’ current recovery level, the Subjective Recovery
Perception Scale (SRP) [28] was utilized.
The SRP has reference values from 0 to 10 in the extremes, meaning
“extremely tired” and “very well recovered,”
respectively.
Statistical analysis
The normality of the data was verified by the Shapiro–Wilk test. For
melatonin response, two-way analysis of variance (ANOVA) repeated measurements
followed by Bonferroni’s post hoc test was conducted to assess the
interaction between time and intervention. For the other variables, the
Student’s paired t-test or Wilcoxon test was performed to verify
differences between interventions. In addition, to verify correlation between
melatonin differences and cognitive and performance parameters, a Spearmanʼs
test was performed. IBM SPSS statistical software (Version 20; IBM Corp.,
Armonk, NY, USA) was used to perform data analyses. The level of significance
adopted was p<0.05. Cohen’s effect size (ES) were
calculated and magnitude was classified as:<0.2=trivial,
0.2–0.6=small, 0.6–1.2=moderate,
1.2–2.0=large, and>2.0=very large [29].
Results
Using a post-hoc statistical power test with 15 participants, a power of 0.76 was
reached [G* Power Software (Dusseldorf, Germany); statistical
test=ANOVA: repeated measures, within-between interaction;
α=0.05; ES=0.37; number of groups=2; and number of
measures=2]. All volleyball players were exposed to the same illuminance
level (blue lenses, 352.4±35.9 lux; transparent lenses, 349.2±35.1
lux; p=0.834, ES=0.09).
There was no time effect [F (1,14)=0.035, p=0.854) or interaction
between time and intervention [F (1,14)=3.576, p=0.08] for
melatonin. In both conditions [(blue lens, pre: 0.79±0.73 to post:
1.19±1.37 (pg/dL), p=0.252, ES=0.38; colorless lens,
pre 0.97±1.00 to post 0.67±0.71 (pg/dL), p=0.305,
ES=−0.35], post hoc analysis showed that melatonin did not change
within and between groups ([Fig. 3]).
Fig. 3 Melatonin responses before and after exposure of the blue and
transparent lenses. Data are expressed as individual values±SD.
There were no changes between blue and transparent lenses on the Karolinska
Sleepiness Scale (Blue, 4.2±1.7 KSS; colorless, 4.9±1.2 KSS;
p=0.148; ES=–0.48), although a small ES was found, and the
Visual Analogue Mood Scale ([Table 1]).
Table 1 Visual Analogue Mood Scale: results between blue and
transparent lenses.
|
Blue lens
|
Transparent lens
|
Pvalue
|
ES
|
|
Anxiety (mm)
|
177.0±30.0
|
168.0±19.0
|
0.589
|
0.17
|
|
Physical sedation (mm)
|
315.0±80.0
|
327.0±66.0
|
0.532
|
0.05
|
|
Mental sedation (mm)
|
107.0±17.0
|
110.0±34.0
|
0.875
|
0.20
|
|
Other feelings (mm)
|
165.0±40.0
|
177.0±47.0
|
0.570
|
−0.17
|
Median±interquartile range; effect size, ES.
Concerning ANT, there was no significant difference in the number of correct answers
(p=0.308) between blue and transparent lenses. The same was observed to time
reaction (p=0.698) and T-test performance (p=0.066). However, as for
T-test performance, a small ES was found. These results are displayed in [Fig. 4].
Fig. 4
a, Accuracy results (p=0.308, ES=–0.47);
b, Reaction time (p=0.698, ES=–0.14);
c, T-test (P=0.066; ES=0.41). Data are expressed
as individual values±SD.
A negative significant correlation was observed between melatonin differences and
physical sedation for glasses with blue lenses. No other significant correlations
were found ([Table 2]).
Table 2 Correlation between melatonin differences and alertness,
cognitive and performance parameters for glasses with blue and
transparent lenses.
|
Glasses with blue lenses
|
Glasses with transparent lenses
|
|
Melatonin differences
|
Melatonin differences
|
|
Iluminance level
|
–0.432
|
–0.179
|
|
KSS
|
0.002
|
0.002
|
|
Anxiety
|
0.434
|
0.083
|
|
Physical sedation
|
–0.526*
|
–0.034
|
|
Mental sedation
|
–0.144
|
0.296
|
|
Other feelings
|
0.066
|
–0.091
|
|
ANT
|
0.378
|
0.381
|
|
Reaction time
|
0.020
|
0.168
|
|
T-test
|
–0.429
|
–0.076
|
KSS, Karolinska Sleepiness Scale; ANT, Attentional Networks Test;
*p=0.044
Discussion
Although a previous investigation evaluated the acute effects of wearing colored-lens
glasses on exercise performance and testosterone concentration [30], in this study we tested the effects of
wearing blue lenses under artificial light conditions in melatonin responses, and
physical and cognitive performance. Overall, we found no significant differences
when assessing salivary melatonin, alertness, mood, and performance variables
(reaction time and T-test).
In contrast with previous investigations [6]
[7]
[31]
[32]
[33], light exposure in our
study did not reduce salivary melatonin level. However, these studies used a longer
period than 30 minutes, suggesting that the time of exposure to light may
influence the outcome.
Some studies have shown a correlation between subjective alertness and measures of
cognitive performance [34]
[35]. In light of these results, Zhou et al.
[36] proposes that the subjective
alertness may not reflect an improvement in cognitive performance and vice versa,
and that a reduction in alertness does not always reflect some impairment in
cognitive performance tasks. The present study does not present a significant
difference in the alertness level, nor did it demonstrate improved cognitive
performance from the use of blue lenses, given the lighting condition. However,
other studies demonstrated that exposure to light caused changes in both objective
and subjective measures of alertness and improvements in cognitive performance [7]
[37]
[38]
[39].
An important aspect of our results is that a complex psychomotor test was used to
analyze the objective measure of cognition. No relevant changes were observed in the
reaction time with the use of lenses with filters and without filters. These results
may be due to the non-specificity of the test. The reaction time reflects the nature
and duration of the cognitive processes, intervening in successive stages of
information processing, between presentation of the stimulus and the response [40]. This proves to be of fundamental
importance for volleyball players because, suddenly, there is a need for wide
attention to other situations that require selective attention before analyzing the
specific stimulus and selecting the appropriate action.
During this experiment, the illuminance pattern was kept constant, using 12
fluorescent lamps with a color temperature of 4,100 kelvin, generating a total lux
measurement of ~ 822 lux, and at eye level in the vertical plane, a
measurement of ~ 352 lux. According to Cajochen et al. [41], exposure to a lighting pattern of around
90 to 180 lux is sufficient to promote changes in objective and subjective measures
of alertness as well as melatonin suppression. It is important to note that the
research by Cajochen et al. [41] used the
Constant Routine method, in which all individuals were systematically controlled in
relation to the pattern of sleep, awakening, food and fluid intake, which was not
possible with the same technique in our work.
Regarding the lack of positive findings under the effect of
alertness/drowsiness, it is important to mention the work of Souman et al.
[42], who carried out a systematic review
on the acute effects of the state of alertness upon exposure to light in a review of
publications of the last 26 years (1990–2016). It was found that most
studies reported significant differences in subjective alertness; however 17 of 45
studies (38%) were unable to find a significant effect. This can be
justified by a set of factors such as chronotyping, circadian phase, history of
previous light, and genetic factors. We were able to control only the chronotypology
of the participants, who were classified as indifferent chronotypology
(47.8±9.9), and the circadian phase, with all evaluations performed in the
same night period after 18:00, when the melatonin production threshold was reached
in low light [24].
Regarding the state of mood, it was expected that through the use of lenses with
filters, the subjective state of alertness would be changed, and consequently the
state of mental sedation (alert and attentive). We ascertained only an ES of 0.2 for
mental sedation. Leichtfried et al. [43],
unlike us, observed changes in the state of subjective mood when healthy individuals
were exposed to an illumination of 6,500 kelvin in the morning. It is important to
note that this color temperature pattern has a band of short length (blue spectrum),
unlike the color temperature of 4,100 kelvin. This is of fundamental importance
because non-visual reception is more sensitive to short wavelength bands compared to
cones and rods, which are more sensitive to medium-length waves and may have
influenced subjective mood. The authors also performed stimulation in the daytime,
as this subjective measure was more conducive to changes in the morning.
Our objective was to observe whether stimulation during the night would modify
biological circadian changes (acute melatonin suppression and subjective alertness)
in conjunction with psychometric changes (subjective mood), especially in relation
to mental sedation; however this it did not occur in our study. On the other hand,
Plitnick et al. [44] demonstrated that night
exposure to lighting with short (blue) or long (red) waves caused positive changes
in the subjective measures of alertness and mood, regardless of the decrease in
salivary melatonin levels. In our study, no significant correlation was found
between melatonin differences and mental sedation and KSS for both glasses with blue
and transparent lenses. This can demonstrate a dissociation of patterns of changes
in circadian biological measures and psychological status. However, a significant
negative correlation was found between melatonin differences and physical
sedation.
One of the limitations of our work is related to records of sleep quality. No
actigraphy measures were used to record normal sleep periods or periods of insomnia
during the study. The amount and time of exposure to light is another factor that
may have interfered with the results. In a clinical trial, an interesting result was
identified that may point to improvements in the research methodology on the effects
of exposure to light and suppression of melatonin. It has been found that exposure
to blue light can increase subjective alertness, but did not influence objective
alertness [45]. An important aspect of these
data is that Hanifin et al. [45] and other
authors used more than 30 minutes of exposure to light [6]
[7]
[33]. Finally, although we
measured the T-test time with a stopwatch, the considered difference measurement
error between evaluators was no more than 3%.
