Yasukouchi et al. Journal of Physiological Anthropology
https://doi.org/10.1186/s40101-019-0203-4(2019) 38:10

ORIGINAL ARTICLE

Open Access

Non-visual effects of diurnal exposure to an
artificial skylight, including nocturnal
melatonin suppression
Akira Yasukouchi1*, Takafumi Maeda1 , Kazuyoshi Hara2 and Hiroyuki Furuune2

Abstract
Background: Recently, more consideration is being given to the beneficial effects of lighting on the maintenanceand promotion of the health and well-being of office occupants in built environments. A new lighting technology
using Rayleigh scattering has made it possible to simulate a blue sky. However, to date, no studies have examined
the possible beneficial effects of such artificial skylights. The aims of this study were to examine the non-visual
effects of artificial skylights and conventional fluorescent lights in a simulated office environment and to clarify thefeature effects of the artificial skylights.
Methods: Participants were 10 healthy male adults. Non-visual effects were evaluated based on brain arousal levels(α-wave ratio and contingent negative variation [CNV]), autonomic nervous activity (heart rate variability [HRV]),
work performance, and subjective responses during daytime exposure to either an artificial skylight or fluorescentlights, as well as nocturnal melatonin secretion.
Results: Subjective evaluations of both room lighting-related “natural” and “attractive” items and the “connected tonature” item were significantly higher with the skylight than with the fluorescent lights. Cortical arousal levels
obtained from the early component of the CNV amplitude were significantly lower with the skylight than with thefluorescent lights, whereas α-wave ratio and work performance were similar between the two light sources. The
HRV evaluation showed that sympathetic nerve tone was lower and parasympathetic nerve tone was higher, bothsignificantly, for the skylight than for the fluorescent lights during daytime. Nocturnal melatonin secretion was
significantly greater before and during light exposure at night under the daytime skylight than under the fluorescentlights.
Conclusions: Our results suggest that artificial skylights have some advantages over conventional fluorescentlights in maintaining ordinary work performance during daytime with less psychological and physiologicalstress. The findings also suggest that the artificial skylights would enable built environments to maintain longterm comfort and productivity.
Keywords: Artificial skylight, Fluorescent light, Office, Non-visual effect, Arousal level, Melatonin secretion* Correspondence: physioanthropol@yahoo.co.jp
1
Department of Human Science, Faculty of Design, Kyushu University, 4-9-1,
Shiobaru, Minami-ku, Fukuoka 815-8540, Japan
Full list of author information is available at the end of the article© The Author(s). 2019 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0
International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, andreproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to
the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver(http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.

Yasukouchi et al. Journal of Physiological Anthropology(2019) 38:10

BackgroundPhysiological anthropology is a field that primarily focuseson humans living in modern society. This is because
humans are considered to be adapted to ancient environments, given that we spent nearly all of our history as a
species in such environments, dating back to our huntergatherer stage. Thus, there must be a discrepancy between
environments to which humans have already adapted andenvironments to which humans are struggling to adapt
when seeking comfort and convenience. For this reason,
physiological anthropologists study the environmentalpressures on humans in highly technological environments [1–6], in which lighting must also be examined as
one of the physical factors affecting environmental adaptability [7–9].
Windows give occupants in built environments feelings of connectedness to nature and help them to feel
more relaxed, which boosts their health and well-being[10–13]. However, occupants in windowless environments or those who are situated away from windows
have to depend on artificial light and have no connection to nature. Natural light and views from windows
also enhance attention and cognition [14], whereas windowless environments or those with relatively low artificial light illumination negatively affect arousal level andproductivity due to sleepiness [15]. Furthermore, there isno time-dependent variation in natural light and views
throughout the day in windowless environments, which
can increase psychological stress [16] and lower job satisfaction [17].
In addition to more advanced lighting technologies suchas light-emitting diodes (LEDs) and organic electroluminescence (OEL), a new technology has been developed tosimulate natural sunlight and blue skies. This system iscomposed of a light source and nanostructured material
that recreates the Rayleigh scattering phenomenon that
occurs naturally in the Earth’s atmosphere whereby
shorter wavelengths of light are strongly diffused while
longer wavelengths are weakly diffused. This phenomenon
can be recreated in a small box and helps people to perceive a realistic sun and blue sky with indefinite depth,
even though the spectral distribution is not exactly thesame as that of natural sunlight. The introduction of thisartificial skylight in a windowless environment or space
situated away from windows gives the impression of connectedness to nature, which should boost psychologicaland physiological well-being and health.
Canazei et al. [18] determined, using a questionnairebased approach, that an artificial skylight similar to the oneused in this study had beneficial effects compared with conventional fluorescent lighting. However, in addition to psychological effects, physiological responses caused by nonvisual effects or non-image-forming effects should also be
considered to improve worker health, well-being, andPage 2 of 12

productivity [15, 19–23]. Ours is the first study to examine
the non-visual effects of artificial skylights in a simulated office environment. The existence of non-visual effects was
suggested when the retinohypothalamic tract was first identified [24] and, as a non-visual effect, light-induced nocturnal melatonin suppression was first reported by Lewy et al.
[25]. Brainard et al. [26] and Thapan et al. [27] have reported the peak spectral sensitivity of light-induced melatonin suppression at night, with the peak located at aroundthe blue part of light, which was soon further supported by
the discovery of intrinsically photosensitive retinal ganglion
cells (ipRGCs) [28]. There are several important neural sites
from the retina to the pineal body via the hypothalamus
that are related to circadian rhythms, autonomic nervous
activity, endocrine secretion, cortical arousal level, andmuscle tension. These are called “non-visual effects” andare associated with signal input to ipRGCs projected to theretinohypothalamic tract. The intensity, wavelength,
duration, timing, and pattern of light are important factors
affecting photic resetting of non-visual effects [29], particularly in terms of the circadian system and suppression ofmelatonin secretion. Thus, changes in physiological function caused by the light environment would affect the productivity, comfort, and well-being of office workers, not only
as an acute effect, but also as a long-term effect.
The aims of this study were to examine the corticalarousal level, autonomic nervous activity, and nocturnal
melatonin secretion, as well as work performance andsubjective evaluations, of participants exposed to artificial skylights and conventional fluorescent lights in asimulated office and to clarify the feature effects of theartificial skylight.

Methods
ParticipantsParticipants were 10 healthy male college students(mean age ± standard deviation, 23.6 ± 0.7 years). They
were enrolled in the study after completing a psychological interview and screening for sleep disorders, medication use, recent night work, and recent overseas travel
across time zones. This study was approved by the Ethics Committee of the Faculty of Design, Kyushu University. Written informed consent was obtained from allparticipants prior to enrollment. Participants were required to keep a regular 8-h sleep schedule for at least 5days before the experiment, which was confirmed fromtheir daily sleep diary and actigraph data (Actiwatch-L;
Mini-Mitter Co., Inc., Bend, OR). Participants were also
asked to refrain from using caffeine, nicotine, and alcohol for 1 day before the experiment.
Experimental protocol

A climatic chamber at the Faculty of Design, Kyushu
University, was used for our study from November toYasukouchi et al. Journal of Physiological Anthropology(2019) 38:10

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Fig. 1 On the left is the room with artificial skylights; the room with fluorescent lights is shown on the right

December 2017. Two simulated office environmentswith identical areas (width, 1960 mm; length, 2700 mm;
height, 1940 mm) were set up as shown in Fig. 1. The
ambient temperature in each room was kept at 25 ±0.04 °C with 50 ± 0.10% relative humidity. The artificial
skylight room (CoeLux ST, CoeLux S.r.L) is shown on
the left side of the figure, and the fluorescent lightroom
is shown on the right side.
The lighting conditions of illuminance and correlatedcolor temperature (CCT) when horizontal illuminance
was set as the same light intensity measured on the desk
(440 lx) are described in Table 1. The work plane is where
the most important tasks in the office occur and because
the relevant guidelines in each country generally recommend that horizontal illuminance be measured on thedesk. The melanopic illuminance obtained from corneal
spectral irradiance measures calculated by the equation ofLucas et al. [30] determined 4.99 melanopic lx with skylights, which is about 35% greater than the 3.70 melanopiclx with fluorescent light. In addition, cyanopic, rhodopic,
chloropic, and erythropic illuminances were also 22%,
22%, 8%, and 0% higher with skylights, respectively. An
artificial skylight installed on the ceiling is shown in Fig. 2,
Table 1 Lighting conditions of the artificial skylights andfluorescent lights
Measured on the desk

Measured at the cornea

Illuminance (lx)

CCT (K)

Illuminance (lx)

CCT (K)

AS

440

6700

417

6200

FL

440

4800

330

4800

CCT correlated color temperature, AS artificial skylight, FL fluorescent lights

and the spectral power distributions of the skylights andfluorescent lights are shown in Fig. 3.
All measurement items and timings are shown in Fig.
4, and the schedule is shown in Fig. 5. Participants arrived at the laboratory before 09:00 and wore t-shirts
and shorts to homogenize the thermal conditions.
After we set the sensors to take measurements until
09:50, a subjective evaluation of the lighting in the room
was made and the first task was started. The simulated
office tasks consisted of a contingent negative variation(CNV) paradigm and arithmetic task. Both tasks were
performed for 36 min with a 3-min rest and interval as asingle set (Fig. 4). The set of tasks was performed twice
each morning and twice each afternoon (Fig. 5). The
measurement timing of α-wave ratio, heart rate variability (HRV), and subjective evaluation in a set are shownin Fig. 4. The early component of the CNV amplitude
and the selective response time were measured during
the CNV paradigm, and the number of answers and ratio of correct answers were obtained during the arithmetic task. HRV was not measured before the CNV
paradigm in the second set each morning and afternoon.
As shown in Fig. 5, the first and the second task sets
started at 10:00 and 11:00 in the morning and at 13:30
and 14:30 in the afternoon, respectively, and were
followed by free time until 18:00. The participants werepermitted to read books, watch videos, and play games
that did not overly stimulate them during the free-time
periods after 15:00 and 19:00. The display was set to 0.5
cd/m2 during the videos and games so as to not affectmelatonin secretion, in accordance with Higuchi et al.
[31]. The lighting conditions were as shown in Table 1

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Measurements
Contingent negative variation

Fig. 2 Artificial skylight seen from below

during the period of office simulation from 10:00 to 18:00, set at 200 lx measured on the desk for each lightsource shown in Table 1 during the evening meal andshower period from 18:00 to 19:00, and emitted dim
light (< 30 lx) during the free period from 19:00 to 00:
00. Saliva samples were taken five times (solid circles inFig. 5) from 23:00 to 01:30 for analysis of melatonin concentrations. During this period, participants, regardlessof the light source condition during the daytime, were
exposed to fluorescent light at 500 lx and 5000 K from
00:00 to 01:30 to evaluate light-induced nocturnal melatonin suppression. The experiment was then finished.

CNV, one of the event-related potentials in the brain,
was first proposed by Walter et al. [32], who suggested
that the components of the CNV reflect attention and
expectancy related to a given stimulus as well as arousal
level as a fundamental function. In this study, electroencephalography (EEG) was measured by using AgAgCl disc electrodes located at Fz, Cz, and Pz according to the International 10–20 system with a linked ear
reference (Polimate AP1000; TEAC Co. Ltd., Tokyo,
Japan). The CNV paradigm consisted of a warning
stimulus (S1) and imperative stimulus (S2) with a 2500ms interval. The paradigm was repeated 44 times over
15 min, and the inter-trial interval was randomized between 8000 and 12000 ms. The baseline level was obtained from the averaged potential during the 500 ms
before the S1 stimulus. The arithmetic mean of theearly component of the CNV amplitude was obtained
between 500 and 1000 ms from the S1 stimulus during
the 44 trials with the exclusion of trials with noise due
to eye movement or blinking detected by electrooculography. The imperative stimulus (S2) consisted of a red
circle as a target signal and a green circle as a nontarget signal; target signals appeared in 70% of all trials.
The participant was requested to push the button assoon as possible upon recognizing the red signal of S2
on the display.

Fig. 3 Spectral distribution of the artificial skylight (upper) and the fluorescent lights (lower). The wavelength region around 480 nm is where
photoreceptors related to non-visual effects (ipRGCs) have high sensitivity. AS, artificial skylight; FL, fluorescent lights

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Fig. 4 Simulated office work during daytime

Work performance

Heart rate variability

The average response times of correct answers during
the CNV paradigm were considered measures of work
performance. The arithmetic task was a single-digit
addition task in which participants were asked to add
neighboring digits for 12 min, and the number of answers and the ratio of correct answers were considered
to reflect work performance.

Participants were asked to gaze at a point on the wall infront of them. Their breathing was controlled at 15
breaths/min with a metronome while an electrocardiogram (ECG) from bipolar chest leads was measured before
the CNV paradigm and after the arithmetic task in eachtask set (Fig. 4). The R-R interval obtained from the ECG
data was analyzed by the FFT technique with MaP1060
Ver5.16 (Nihonsanteku Co., Ltd., Osaka, Japan) and thepowers of the high-frequency (HF, 0.20–0.35 Hz) andlow-frequency (LF, 0.05–0.20 Hz) components were calculated to evaluate autonomic nervous activity. LF/HF
and HF/(LF + HF) were considered measures of sympathetic and parasympathetic nerve tones, respectively.

α-wave ratio

The spontaneous EEG at Pz was measured for 3 min(Fig. 4) with participants at rest sitting on a chairwith their eyes open. The power values for the αwave (8–13 Hz) and β-wave (13–30 Hz) were calculated from a fast Fourier transform (FFT) of the EEG
data. The relative power value of the α-wave (α-wave
ratio) was obtained by αrp/(αrp + βrp) × 100, where
rp is the relative power value.

Melatonin

Saliva samples obtained with a Salivette saliva collector(Sarstedt AG & Co. KG, Sarstedt, Germany) were

Fig. 5 Schedule lighting conditions: 09:00–18:00: artificial skylight or fluorescent lights (see Table 1). 18:00–19:00: artificial skylight or fluorescentlights with 200 lx and CCT kept the same; 19:00–00:00: dim light condition (< 30 lx). 00:00–01:30: fluorescent lights with 500 lx and 5000 K

Yasukouchi et al. Journal of Physiological Anthropology(2019) 38:10

centrifuged for 5 min at 1000 rpm, frozen at – 20 °C,
and stored until assayed. The melatonin levels of thesamples were analyzed in duplicate with a commerciallyavailable RIA kit (Direct Saliva Melatonin RIA, RKDSM2; BÜHLMANN Laboratories AG, Schönenbuch,
Switzerland). The mean values of duplicates were used
for the analysis.
Subjective evaluation

The questionnaires used in Canazei et al. [18] consisted of
38 room atmosphere-related terms, 6 room lightingrelated terms, and 1 item of connectedness to nature. Our
study used the same questionnaires of room lighting andconnectedness to nature and selected 7 of the 38 room
atmosphere-related terms that were found to have significant main effects of light and time by Canazei et al. [18].
Each questionnaire is detailed below.
1) Room atmosphere questionnaire Seven atmosphere terms: threatening, cozy, active,

uninhibited, safe, restless, and stimulating Rating categories and numerical coding: 1, “not atall”; 2, “a little”; 3, “rather”; and 4, “very”

2) Room lighting questionnaire Six room lighting-related bipolar adjective pairs:

pleasant–unpleasant, attractive–unattractive, natural–artificial, glary–without glare, dim–bright, andeven–uneven Numerical ratings from 0 to 53) One item of connectedness to nature Rating categories and numerical coding: 1, “not atall”; 2, “a little”; 3, “rather”; and 4, “very”

Statistical analysis

Types of light sources and time were used as the variables of each measurement, and two-way analyses ofvariance were conducted with Bonferroni correction
using SPSS version 22.0 (SPSS Inc., Chicago, IL, USA).
Differences with p < 0.05 were considered statisticallysignificant.

Results
Brain arousal levels were evaluated by the α-wave ratio
and early component of the CNV amplitude. The α-wave
ratio is the arousal level in response to non-specific multiple stimuli around participants, whereas the early component of the CNV amplitude is the arousal level in
response to specific serial events occurring during thePage 6 of 12

CNV paradigm. There was no significant difference inthe α-wave ratio between the two lighting conditions.
However, the early components of CNV amplitudes weresignificantly higher at both Fz and Cz under the fluorescent lights than under the artificial skylights (Fig. 6; both p< 0.05) with no time variation.
Regarding productivity measured by the arithmetic
task (number of answers and ratio of correct answers)
and the selective response time obtained during the
CNV paradigm, no significant differences were observed.
Lighting conditions had no effect on calculation andselective response times that required concentration,
cognition, and judgment.
Sympathetic and parasympathetic nervous activities,
obtained from HRV, are shown in Figs. 7 and 8, respectively. Sympathetic nervous activity for overall measurements in the daytime indicated significant effects of lightand time (both p < 0.01). Although there was no difference before the first task in the morning (morning 1 inFig. 7), the difference became more evident throughout
the day and was still present at afternoon 1 before thefirst task in the afternoon, showing that the activity wassignificantly lowered under the artificial skylights compared with the fluorescent lights. In the same manner, asignificant effect of light on parasympathetic nervous activity was observed, with enhanced activity under theartificial skylights compared with the fluorescent lights(p < 0.05).
Nocturnal melatonin secretion is shown in Fig. 9.
Melatonin secretion was measured at 23:00 and 23:50
under dim light conditions (< 30 lx) and at 00:30, 01:00,
and 01:30 during fluorescent light exposure (500 lx and5000 K). There were significant main effects of light andtime (both p < 0.05). According to multiple comparisonsby Bonferroni correction, no difference was found at 23:
00. However, a significant difference was evident 50 min
later under dim light conditions; this difference wasgreater in the participants exposed to the artificial skylights (p < 0.05). The difference continued even during
light exposure.
Regarding subjective responses, there were significant
main effects of light both for room lighting-related
effects and connectedness to nature, showing that participants considered artificial skylights more “attractive”
and more “natural” (both p < 0.01) than fluorescentlights among the 6 items of room lighting-related effects
and also felt greater connectedness to nature (p < 0.01)(Fig. 10). However, no significant changes with time
were found for all three subjective items.

Discussion
There has been considerable focus on green construction and environmental sustainability in recent years,
but relatively little attention has been paid to biologicalYasukouchi et al. Journal of Physiological Anthropology(2019) 38:10

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Fig. 6 Early component of CNV amplitude obtained from Fz and Cz. A larger negative value indicates higher arousal. AS, artificial skylight; FL, fluorescent lights

sustainability. Studies of occupant health and well-being
in built environments have recently begun to be conducted. It has been demonstrated that light, in addition
to air and water quality, has a strong effect on occupant
health and well-being in offices, not only from the pointof view of subjective comfort, but also in terms of objective aspects or non-visual effects [15, 19, 21, 33].
A newly developed artificial skylight was examined
here, although its spectral distribution is not exactly thesame as that of natural sunlight. The aims of this study
were to examine the non-visual effects of lights in termsof cortical arousal levels, autonomic nervous activity,and nocturnal melatonin secretion; to examine work
productivity and subjective responses during diurnal

exposure to artificial skylights and conventional fluorescent lights in a simulated office environment; and toevaluate whether artificial skylights should be favored in
offices from the viewpoint of adaptability to diurnal
light.
Subjective evaluations

Canazei et al. [18] were the first to perform subjective
comparisons of artificial skylights and fluorescent lights
in a simulated office environment built in a laboratory.
They investigated subjective responses by using questionnaires, which consisted of 38 room atmosphererelated terms, 6 room lighting-related terms, and 1 itemof connectedness to nature. Our study used the same

Fig. 7 Sympathetic nervous activity (LF/HF) evaluated from heart rate variability (HRV). AS, artificial skylight; FL, fluorescent lights

Yasukouchi et al. Journal of Physiological Anthropology(2019) 38:10

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Fig. 8 Parasympathetic nervous activity (HF/(LF+HF)) evaluated from heart rate variability (HRV). AS, artificial skylight; FL, fluorescent lights

items related to room lighting and connectedness to nature, and we selected 7 of the 38 room atmosphererelated items found to exert significant main effects oflight and time in Canazei et al. The artificial skylight intheir study was much larger size and had a higher
light intensity (1215 lx) than our skylights (400 lx).
However, significant effects were obtained from two
terms—“attractive” and “nature”—of perceived room
lighting and connectedness to nature; no significant
effects were found among the 7 items of perceived
room atmosphere in our study. Thus, these 3 items
showing significant effects even at around 400 lx
might be the features of skylights that mimic natural
blue sky with indefinite depth.

Non-visual effects

Many studies have focused on the non-visual effects oflight [7, 34], with application studies showing its beneficial effects on human health and well-being and how totranslate scientific findings into office lighting designs[15, 19, 22, 23]. Non-visual responses include almost allphysiological functions affected by photic signals from
ipRGCs projected to all major retinorecipient parts ofthe brain [28, 35]. From the viewpoint of the relationshipbetween CCT and the action factors on circadianrhythm, non-visual responses are dependent on thephysical nature of light, such as its intensity and spectral
distribution, and not on the type of light source [36]. In
this study, the non-visual effects of artificial skylights

Fig. 9 Nocturnal melatonin secretion during dim light and light exposure. AS, artificial skylight; FL, fluorescent lights

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Fig. 10 Subjective evaluations. The vertical scales of left and middle graphs show that lower values are more attractive and natural, respectively,
and the right shows that higher value indicates more connectedness. AS, artificial skylight; FL, fluorescent lights

and conventional fluorescent lights during daytime exposure were evaluated in terms of arousal level, autonomic nervous activity, and nocturnal melatoninsecretion. The arousal level of the brain and autonomic
nervous tension are thought to be associated with office
work productivity. Nocturnal melatonin secretion, which
affects sleep and circadian rhythms, might also have anindirect effect on long-term work performance.
Regarding arousal levels, the α-wave ratio and the early
component of the CNV amplitude were measured. The
α-wave ratio was obtained from spontaneous EEG
immediately before the CNV paradigm was begun andrepresents the arousal level with non-specific stimulation. The CNV, one of the event-related potentials, hasbeen evaluated as expectancy [32] and motivation [37]while the early component of the CNV amplitude has
been considered the attention or arousal level with thespecific serial events of warning, imperative stimuli, andbutton pushing [38]. This type of attention is called thephasic arousal level [39].
Arousal level or alertness

There have been many studies on alertness involving
spontaneous EEG evaluation. These studies have reported that alertness and cortical arousal level were increased by monochromatic blue light and daylight with ahigher CCT in the case of polychromatic light [40–44].
However, there was no significant difference in the αwave ratio in this study between the two types of lightstudied, showing that the arousal level with non-specificstimulation was the same under the two lighting
conditions.
On the other hand, the early component of the CNV
amplitude or the phasic arousal level measured at both

Fz and Cz was significantly lower under the artificial
skylights than under the fluorescent lights. It might be
speculated that the artificial skylights would increase
sleepiness and lower work productivity. However, there
was no increase in the α-wave ratio reflecting increased
sleepiness. In addition, the work performance of the selective response time and arithmetic task did not reveal
any differences between the two lighting conditions. The
results suggested that individuals under artificial skylights would have the same work performance as thoseunder fluorescent lights with a lower phasic arousal leveland same sleepiness. In other words, occupants under
the fluorescent lights might have a slightly excessive tension of arousal level when achieving the same work compared with those under the artificial skylights [45].
Deguchi and Sato [46] were the first to report theeffect of the CCT of light on the early component of the
CNV amplitude. They evaluated phasic arousal levels
under CCT conditions of 3000 K, 5000 K, and 7500 K
with the same light intensity of 1000 lx. Their results
showed that the simple response time during the CNV
paradigm was not significantly affected by CCT while
the early component of the CNV amplitude was significantly higher with 7500 K of light than with 3000 K.
Subsequent studies also demonstrated the same results,
namely, that a higher CCT enhanced the arousal levelobtained by the CNV paradigm [47, 48]. Because
ipRGCs are sensitive to short-wavelength light, a higher
CCT with a greater amount of short-wavelength energy
is considered to strengthen non-visual effects [26, 28,30]. However, the phasic arousal level was significantly
lower in the participants exposed to the artificial skylights than in those exposed to the fluorescent lights,
even though the CCT and light intensity of the lights

Yasukouchi et al. Journal of Physiological Anthropology(2019) 38:10

measured at the occupant’s eye level in this study werehigher under the artificial skylights (417 lx with 6200 K)
than under the fluorescent lights (330 lx with 4800 K).
This finding is different from that of other studies.
The non-visual effect is supposed to be affected not
only by direct photic stimulation to ipRGCs, but also integrated signal input to ipRGCs from rods and S, M, andL cones in the retina. Because the integration process is
not completely understood, the non-visual effects cannot
be entirely explained by the intensity and CCT of light[30, 49]. It is implied that the results obtained with thedifferent spectral distributions of the two light sourcesin this study could be affected by differences in the integration of signals from the five photic receptors. However, the cause is unknown. Different types of lightsources were used in this study, whereas many otherstudies have used the same light source to specificallyexamine CCT and/or intensity effects. Therefore, one ofthe possible reasons for the conflicting result might be
an impact on the psychological impression of the artificial skylights. Non-visual effects were originally considered independent of visual effects or psychologicalresponses to a view, and yet, it was very likely that thepsychological effects of an artificial skylight with no glare
and indefinite depth would reduce tension to a moderatelevel because the skylight gives occupants comprehensive feelings of a natural atmosphere and connectedness
to nature. Thus, although the differences in the lightsources themselves were believed to not affect nonvisual effects [36], the impact on psychological factors
might overlap with non-visual functions.
Autonomic nervous activity

In terms of autonomic nervous activity, it is generally
considered that sympathetic nerve tone becomes dominant over parasympathetic tone immediately before andduring mental work. Some studies have examined the effect of the CCT of light on autonomic nervous activity
evaluated using HRV [50, 51], blood pressure [52], andbody temperature [53, 54]. These studies reported that ahigher CCT strengthens sympathetic nerve tone or both
sympathetic and parasympathetic tones.
In the case of autonomic nerve tone related to cardiac
activity evaluated from HRV in this study, the sympathetic nerve tone (LF/HF) was significantly lower under
the artificial skylight conditions than under the fluorescent light conditions when all of the daytime data werepooled (p < 0.01; Fig. 7). Similarly, parasympathetic tone
(HF/(LF+HF)) was significantly higher with the skylight(p < 0.05; Fig. 8). Thus, our results indicate lower autonomic nerve tension with the skylight than with thefluorescent lights, despite the higher CCT of the skylight. This finding also conflicts with those of previous
reports. As shown in Figs. 7 and 8, both sympathetic andPage 10 of 12

parasympathetic nerve tones were similar for the two
types of lights immediately before the first CNV paradigm at morning 1; the tension in the fluorescent light
condition tended to increase at mornings 2 and 3, withthe tendency continuing into the afternoon and being
even higher at afternoon 1 immediately before the CNV
task. These results suggest that consistent work performance with less tension, evaluated not only from the early
component of the CNV amplitude but also from HRV,
could be maintained with the artificial skylights, despite
the higher CCT of skylights. These findings again imply
that differences in psychological responses might haveaffected non-visual effects, as seen in the phasic arousal
level and autonomic nervous activity.
Nocturnal melatonin secretion

Although few studies have investigated the effects oflight exposure in daytime on non-visual responses atnight, these reports have shown effects on nocturnal
melatonin secretion [55], dim light melatonin onset(DLMO) [56], and sleep quality [57, 58]. Therefore, it
was expected that the melatonin secretion measured inthis study would also provide information on how different light sources in daytime affect nocturnal melatoninsecretion, DLMO, and sleep quality. It was anticipated,for example, that the more endogenous the melatoninsecretion before sleep, the greater the improvement insleep quality [59–62]. The increased melatonin secretion
before sleep might be caused by advanced DLMO or secretion activation by light exposure in the morning. The
previous studies showed that the higher intensity and/or
CCT of light in the morning, the smaller the lightinduced nocturnal melatonin suppression and the moreadvanced the circadian rhythm phase (DLMO) [15, 55,63]. There might be psychological effects that occurredin daytime due to different light sources. However, only
fluorescent lights were used at night when melatoninsecretion was evaluated. If any differences in melatoninsecretion were found, they would be caused specificallyby the difference in physical features of light sources
used in the daytime. The melatonin secretion was measured at 23:00 and 23:50 (dim light condition < 30 lx)
before participant exposure to fluorescent lights, withthe results indicating that melatonin secretion at 23:50
was significantly increased under daytime skylight conditions than under fluorescent conditions; no difference inmelatonin secretion between the two lighting conditions
was observed at 23:00 (Fig. 9). Furthermore, the melatonin secretion obtained at 00:30 and 01:00 during light
exposure at 500 lx with 5000 K was found to be significantly higher in those exposed to the artificial skylights
than in those exposed to the fluorescent lights, with thetendency maintained until 01:30 (Fig. 9). The melanopic
illuminance was 4.99 melanopic lx with skylights, which

Yasukouchi et al. Journal of Physiological Anthropology(2019) 38:10

is about 35% greater than the 3.70 melanopic lx withfluorescent light. In addition, cyanopic, rhodopic, chloropic, and erythropic illuminances were also 22%, 22%, 8%,
and 0% higher, respectively, with the skylight. It is unclear how interactions of photoreceptive units (cones [L,M, S], rods, and ipRGCs) activate ipRGCs. However, thefollowing features of ipRGCs are known [30]: (1) the
fundamental light response is an irradiance-dependent
increase in firing, (2) they are less sensitive than rods orcones (which means greater stimulation is needed), and(3) once the threshold for ipRGC activation has beenreached, activation is remarkably persistent over a long
duration of constant illumination. Therefore, it is speculated that, in addition to higher CCT with skylight, these
integrated photoreceptor inputs might contribute to increased nocturnal melatonin secretion and/or an advanced DLMO shift. Our findings suggest that the use of
artificial skylights in office environments might help tomaintain a constant circadian rhythm when advanced
DLMO actually occurs because the circadian rhythm isgenerally slightly longer than 24 h and ordinary exposure
to light at night additionally delays DLMO. Our results
also suggest that skylights have a weak effect on lightinduced nocturnal melatonin suppression even with ahigher light intensity (500 lx) than that found in ordinary homes. This might also contribute to sleep improvement, as demonstrated by our previous polysomnography
study showing that higher melatonin secretion before
sleep could lead to a longer period of stage 4 sleep [58].

Conclusions
Artificial skylights that simulate natural sunlight and blue
skies help to maintain appropriate work performance withlower psychological and physiological stress and promoteconsistent long-term work productivity with comfort andgood health from the point of view of increased nocturnal
melatonin secretion and/or an advanced shift in circadianrhythms.
Acknowledgements
We have many thanks to Mr. S. Moritsubo, Mr. M. Hayashi, and Ms. S. Shin for
their support to conduct our experiments and are grateful to all theparticipants of this study.

Authors’ contributions
AY contributed to the design of the experiment and wrote the manuscript.
TM performed the experiments and analyzed the data. KH and HF provided
the experimental materials. All authors read and approved the final manuscript.

Funding
Not applicable.

Availability of data and materials
The datasets used and/or analyzed during the current study are available
from the corresponding author on reasonable request.

Page 11 of 12

Ethics approval and consent to participate
This study was approved by the Ethics Committee of Kyushu University(Approval No. 277). All subjects of this study gave their written informed
consent to participate in this study.
Consent for publication
Not applicable.
Competing interests
The authors declare that they have no competing interests.
Author details
1
Department of Human Science, Faculty of Design, Kyushu University, 4-9-1,
Shiobaru, Minami-ku, Fukuoka 815-8540, Japan. 2La Forêt Engineering Co.,
Ltd, Roppongi Annex 7F, 6-7-6, Roppongi, Minato-ku, Tokyo 106-0032, Japan.
Received: 12 June 2019 Accepted: 19 August 2019

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