Higuchi et al. Journal of Physiological Anthropology
https://doi.org/10.1186/s40101-021-00257-x(2021) 40:7

ORIGINAL ARTICLE

Open AccessIs the use of high correlated color
temperature light at night related to delayof sleep timing in university students? A
cross-country study in Japan and China
Shigekazu Higuchi1* , Yandan Lin2*, Jingjing Qiu2, Yichi Zhang3, Michihiro Ohashi3, Sang-il Lee4,
Shingo Kitamura5 and Akira Yasukouchi1

Abstract
Background: Blue-enriched white light at night has the potential to delay the circadian rhythm in daily life. This
study was conducted to determine whether the use of high correlated color temperature (CCT) light at home atnight is associated with delay of sleep timing in university students.
Methods: The survey was conducted in 2014–2015 in 447 university students in Japan and 327 students in China.
Habitual sleep timing and type of CCT light at home were investigated by using a self-administered questionnaire.
The Japanese students were significantly later than the Chinese students in bedtime, wake time, and midpoint of
sleep. They were asked whether the lighting in the room where they spend most of their time at night was closer
to warm color (low CCT) or daylight color (high CCT). The amount of light exposure level during daily life wasmeasured for at least 1 week by the use of a light sensor in 60 students in each country.
Results: The percentages of participants who used high CCT lighting at night were 61.6% for Japanese students
and 80.8% for Chinese students. Bedtime and sleep onset time on school days and free days were significantly laterin the high CCT group than in the low CCT group in Japan. The midpoint of sleep in the high CCT group was
significantly later than that in the low CCT group on free days but not on school days. On the other hand, none of
the sleep measurements on school days and free days were significantly different between the high CCT and low
CCT groups in China. Illuminance level of light exposure during the night was significantly higher in Japanese thanin Chinese, but that in the morning was significantly higher in China than in Japan.
Conclusions: The use of high CCT light at night is associated with delay of sleep timing in Japanese university
students but not in Chinese university students. The effects of light at night on sleep timing and circadian rhythm
may be complicated by other lifestyle factors depending on the country.
Keywords: Circadian rhythm, Sleep, Blue light, Social jetlag, University students, International comparison* Correspondence: higu-s@design.kyushu-u.ac.jp; ydlin@fudan.edu.cn
1
Department of Human Science, Faculty of Design, Kyushu University, 4-9-1
Shiobaru, Minamiku, Fukuoka 815-8540, Japan
2
Institute for Electric Light Sources, Fudan University, Shanghai 200433, China
Full list of author information is available at the end of the article© The Author(s). 2021 Open Access This article is licensed under a Creative Commons Attribution 4.0 International License,
which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give
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Higuchi et al. Journal of Physiological Anthropology(2021) 40:7

Background
Light is a strong synchronizing agent (zeitgeber) for thecircadian system. Light exposure in the morning has animportant role in resetting the circadian rhythm to a 24h cycle in humans. On the other hand, evening light exposure delays the phase of circadian rhythms. This relationship between the timing of light exposure and phaseshift of circadian rhythms is known as a phase-response
curve [1, 2]. In addition to the effect on the circadianrhythm, light at night has an impact on suppression ofmelatonin secretion [3] and an increase in arousal level[4]. These physiological and endocrinological effects are
known as non-visual effects of light and/or non-imageforming effects of light [5–7]. In modern society, mostpeople spend the night safely and comfortably thanks to
artificial lighting at night. However, excessive exposure
to light at night has negative effects on human sleep andcircadian rhythm [8].
The non-visual effects of light depend on the intensityof light, the spectrum of light, and the duration of exposure. The light intensity of even general home lighting
is known to affect circadian rhythms and melatonin secretion [9, 10]. Regarding the light spectrum, blue lightis known to have a strong effect on melatonin suppression and phase shift of the circadian rhythm [11–13]. Intrinsically photosensitive retinal ganglion cells (ipRGCs),
which contain a photopigment called melanopsin, havean important role in the action of blue light [14, 15].
The action of blue light is observed in blue-enrichedpolychromatic light, which is called high correlated color
temperature (CCT) light. High CCT light has greater effects than low CCT light on arousal level and melatonin
suppression [16–18]. The physiological effects of high
CCT of a fluorescent lamp were reported even before
the discovery of ipRGCs [19–22]. In recent years, similar
results have been obtained by exposure to LED lighting[23–25].
However, many of those studies were conducted in alaboratory, and there have been few studies in which the
influence of CCT of home lighting in real life was examined. We previously reported that there were significant
correlations between CCT of lighting at home and phaseof the circadian rhythm in middle-aged adults and primary school children in Japan [26]. The results of thatstudy suggested that high CCT light at home might be acause of the delay of the circadian phase in daily life.
However, that study had a small sample size and there
has been no study conducted in a large population. This
study was conducted in a large number of university students. It is known that university students have the mostnight-typed sleep pattern [27].
Furthermore, the survey was conducted in Japan and
China. Our previous study conducted in Japan showedthat the CCT of lighting used in the home correlates

Page 2 of 8with circadian rhythms [26]. However, sleep habits andcircadian rhythms are influenced not only by the lightenvironment but also by the lifestyle and social environment [28]. It is possible that the CCT of artificial lighting at night has an effect on sleep only in Japan. To
generalize the effect of CCT on sleep in real life, it wasnecessary to research outside Japan. Therefore, thisstudy also included Chinese university students living ina social environment different from that in Japan. We
chose a location where the latitude of the study site,
Shanghai in China, is almost the same as that of Fukuoka in Japan, and the times of sunrise and sunset arealso almost the same.
The purpose of this study was to determine the differences between light environments in China and Japanand to clarify the relationship between light environments at night and sleep habits in university students inJapan and China.

Methods
Participants

The study was conducted in mid-December 2014 andmid-January 2015. The year-end and New Year holidays
were excluded from the survey period. The participants
included 470 Japanese university students and 397 Chinese university students. All participants provided informed written consent. The study was approved by theInstitutional Ethics Committee of Kyushu University; theprotocol and all the procedures were in agreement with
the Declaration of Helsinki. After excluding participants
whose data included typographical errors or for whomdata were missing, we finally analyzed data for 447 Japanese students (average age of 21.5 ± 1.6 years) and 327
Chinese students (average age of 23.3 ± 2.7 years). The
ratio of males in the Japanese students (62.8%) was significantly higher than that in the Chinese students(53.4%).
The survey was conducted in Fukuoka City in Japanand Shanghai City in China. The latitudes of Fukuoka
and Shanghai are 33° 36′ north latitude and 31° 11′
north latitude, respectively. The time difference between
Japan and China is 1 h. The local times at sunrise on
December 1st in Fukuoka and Shanghai were 7:04 and 6:
34, respectively, and the local times at sunset were 17:10
and 16:51, respectively.
Measurements of sleep habits

We investigated the habitual bedtime (lights off time),
subjective sleep latency, and wake time on school days
and free days using a self-administered questionnaire.
The sleep onset time was calculated by adding the sleep
latency to the bedtime, and the sleep period time wascalculated by the time from the sleep onset time to thewake time. The sleep midpoint was calculated by addingHiguchi et al. Journal of Physiological Anthropology(2021) 40:7

1/2 of the sleep period time to the sleep onset time. Social jet lag was calculated as the absolute difference between midpoint of sleep on free days and midpoint onschool days [29].
Table 1 shows the sleep habits of the university students in Japan and China in the present study. After adjustments by sex and age, on both school days and freedays, the Japanese students were significantly later thanthe Chinese students in all measurements including bedtime, sleep onset time, wake time, and midpoint of sleep(p < 0.001). Sleep period time was also significantly
shorter in Japanese students than in Chinese students(p < 0.001). Subjective sleep latency was not significantly different between the two groups. Social jet lag
was significantly larger in Japanese students than in
Chinese students (p < 0.001).
Measurements of light environments

To the best of our knowledge, this study is the firststudy in which the type of CCT light used at home wasinvestigated for a large sample using a questionnaire.
Therefore, we originally made a simple two-choice question. The participants were asked whether the lighting in
the room where they spend most of their time at night
was closer to warm color (low CCT) or daylight color(high CCT). Color photographs were used to help participants make accurate selections. The participants
looked at two photographs of lighting in the
Table 1 Demographic data and sleep habits in university
students in Japan and China
Japanese(n = 447)
21.5 (1.6)

23.3 (2.7)**

Sex, %male

62.8

53.4

**

01:09 (1:01)

23:58 (0:43)***

Sleep habits (school days)

Sleep latency (min)15.1 (11.3)

16.1 (11.7)

n.s.

Sleep onset time

01:24 (1:02)

00:15 (0:46)***

Wake time

08:01 (1:11)

07:42 (0:49)***

Midpoint of sleep

04:43 (0:57)

3:58 (0:39)***

Sleep period time (h)6.62 (1.13)

7.46 (0.90)***

Bedtime

01:37 (1:12)

00:22 (0:51)***

Sleep latency (min)16.0 (13.0)

14.9 (11.7)

n.s.

Sleep habits (free days)

Sleep onset time

01:52 (1:14)

00:38 (0:52)***

Wake time

09:35 (1:25)

08:46 (1:01)***

Midpoint of sleep

05:45 (1:11)

4:42 (0:49)***

Sleep period time (h)7.70 (1.25)

8.15 (0.95)***

1.04 (0.82)

0.73 (0.56)***

Social jet lag (h)Values are means and SD
Adjusted by age and sex **p < 0.01, ***p < 0.001, n.s not significant

questionnaire and chose one of them. The photographs
used in the questionnaire in this study are not allowed
to be shown here due to copyright issues, but almost thesame pictures are shown as a supplemental file (seeAdditional file 1).
In addition, we asked 60 Japanese and 60 Chinese subjects to wear a pendant-type illuminance sensor (HOBO
by Onset UA-002-08) and illuminance was continuously
measured for about 2 weeks at 2-min intervals. After exclusion of subjects due to missing data, data for 54 Japanese students (22.5 ± 2.2 years old, %male = 51.9) and
58 Chinese students (22.6 ± 2.2 years old, %male = 50.4)
were used for analysis. Sleep timing was significantly
later in Japanese students than in Chinese students(Additional file 2).
The resolution of illuminance was about 10 lx. The
subjects were instructed to always wear the sensor while
awake, except for exercise and bathing, and keep it at
the bedside while sleeping. First, the average illuminancewas calculated for each hour to see the change over a
period of 24 h. In another way, illuminance level was
calculated on the basis of habitual bedtime. Average illuminance level for 2 h before habitual bedtime was used
as a representative illuminance level before bedtime athome at night for each participant. Data with 0 lx before
habitual bedtime were excluded from the calculations.
Due to the limited number of measurement days on freedays, the data for free days and school days were notseparated.
Statistical analysis

Chinese(n = 327)

Mean age (years)

BedtimePage 3 of 8

The chi-square test was used for comparison of the percentages of Japanese and Chinese students who use highCCT lighting. As for the illuminance level, a t-test was
conducted after logarithm transformation. A comparisonof sleep measurements between CCT lighting conditions
was made by adjusting age and sex using a linear mixed
model since sleep timing and sleep hours are affected by
sex and age [27]. Pearson’s correlation analysis was performed for illuminance level and sleep measurements.

Results
The percentages of participants who used high CCT
lighting at night were 61.6% for Japanese students and80.8% for Chinese students (Fig. 1). The percentage ofparticipants who used high CCT lighting was significantly higher for Chinese students (chi-square test, p <0.05). Sleep habits were compared in the high CCTgroup and the low CCT group for both Japanese students and Chinese students after adjusting for sex andage. In the Japanese students, bedtime and sleep onset
time in the high CCT group were significantly later thanthose in the low CCT group on school days (p < 0.05)
and free days (p < 0.01) (Table 2). Also, the midpoint ofHiguchi et al. Journal of Physiological Anthropology(2021) 40:7

Page 4 of 8

Fig. 1 Percentages of users of high CCT light and low CCT light at home at night in Japanese and Chinese university students. **p < 0.01

sleep in the high CCT group was significantly later thanthat in the low CCT group on free days (p < 0.01) (Fig. 2).
The wake time on flee days tended to be later (p = 0.052)
and social jet lag (p = 0.051) tended to be larger in thehigh CCT group than in the low CCT group. On theother hand, in the Chinese students, there was no significant difference in sleep measurements between thehigh CCT group and the low CCT group on school days
or free days (Table 3).
Figure 3 shows the changes in illuminance level thatthe participants were exposed to for 24 h. The averageilluminance in the evening (18:00–24:00) was significantly higher for the Japanese students than for the
Chinese students (t = 5.938, p < 0.001). The average illuminance was significantly higher for the Japanese students than for the Chinese students during the period

from midnight to early morning (0:00–6:00) including
sleeping time (t = 7.573, p < 0.001). On the other hand,the average illuminance in the morning (6:00–12:00) was
significantly higher for the Chinese students than for theJapanese students (t = − 2.038, p < 0.05). There was no
significant difference between illuminance levels in theafternoon (12:00–18:00) for the Japanese students andChinese students. The average illuminance level for 2 h
before habitual bedtime was significantly higher for theJapanese students (74.8 lx ± 56.5 lx) than for the Chinese
students (59.9 lx ± 64.8 lx) (t = 7.573, p < 0.01). Correlation analysis was conducted for illuminance level for 2
h before habitual bedtime at night and sleep habits.
There was no significant correlation between illuminance level and any of the sleep measurements for both
the Japanese students and Chinese students.

Table 2 Sleep habits on school days and free days in university
students in Japan

Discussion
The results of investigation of the relation between the
CCT of lighting and sleep habits in each country showedthat bedtime and sleep onset time of the students who
used high CCT lighting was significantly later on schooldays and free days for the Japanese students. Previous laboratory studies showed that high CCT lighting (blueenriched white lighting) has a greater impact on arousal
level and melatonin suppression [16–18]. The present
study supported the results of previous studies conducted in a laboratory, and the association between useof high CCT lighting at night and delay of sleep timing
was confirmed in the real world in a large sample of Japanese students. The delay in sleep timing for users ofhigh CCT lighting in Japan is thought to be caused byan increase in alertness and delay of the circadianrhythm induced by exposure to blue-enriched light before bedtime.
The difference between sleep habits of students in thehigh CCT group and the low CCT group in Japan was
more pronounced on free days than on school days. The
midpoint of sleep on free days was significantly later inthe high CCT group than in the low CCT group (Fig. 2).

High CCT(n = 275)

Low CCT(n = 172)

P value

Bedtime01:13 (0:58)

01:00 (1:04)

0.029 *

Sleep latency (min)15.5 (11.8)

15.1 (10.7)

0.668

Sleep onset time

01:29 (1:00)

01:15 (1:05)

0.026 *

Wake time

08:02 (1:13)

07:59 (1:10)

0.476

Midpoint of sleep

04:45 (0:57)

4:37 (0:59)

0.100

Sleep period time (h)6.55 (1.14)

6.74 (1.11)

0.186

Bedtime01:45 (1:13)

01:25 (1:11)

0.003 **

Sleep latency (min)16.1 (13.3)

16.1 (12.6)

0.949

Japanese
Sleep habits (school days)

Sleep habits (free days)

Sleep onset time

02:02 (1:16)

01:41 (1:11)

0.003 **

Wake time

09:41 (1:27)

09:26 (1:20)

0.052

Midpoint of sleep

05:51 (1:11)

05:33 (1:08)

0.006 **

Sleep period time (h)7.65 (1.33)

7.74 (1.12)

0.495

1.09 (0.84)

0.94 (0.78)

0.051

Social jet lag (h)Values are means and SD.
Adjusted by age and sex *p < 0.05, **p < 0.01

Higuchi et al. Journal of Physiological Anthropology(2021) 40:7

Fig. 2 Box plots of the midpoints of sleep on free days in the highCCT group and the low CCT group in Japanese (left) and Chinese(right). **p < 0.01

Also, wake-up time on free days tended to be delayed in
the high CCT group. Therefore, the delay in the midpoint of sleep on free days might be caused not only by
the delay in bedtime but also by the delay in wake-up
time. It is known that a free day without social restriction tends to reflect individual chronotypes [30].
Table 3 Sleep habits on school days and free days in university
students in China
High CCT(n = 271)

Low CCT(n = 56)

P value

Bed time Bedtime

23:58 (0:43)

00:01 (0:46)

0.790

Sleep latency (min)16.1 (10.4)

16.2 (16.9)

0.891

Sleep onset time

00:14 (0:45)

00:17 (0:48)

0.770

Wake time

07:41 (0:48)

07:45 (0:55)

0.442

Midpoint of sleep

03:58 (0:38)

04:01 (0:43)

0.510

Sleep period time (h)7.45 (0.88)

7.48 (0.92)

0.645

Bedtime00:23 (0:50)

00:22 (0:58)

0.803

Sleep latency (min)14.7 (10.3)

15.7 (16.9)

0.561

ChineseSleep habits (school days)

Sleep habits (free days)

Sleep onset time

00:38 (0:51)

00:37 (0:57)

0.909

Wake time

08:48 (1:02)

08:42 (0:59)

0.508

Midpoint of sleep

04:43 (0:49)

04:40 (0:53)

0.633

Sleep period time (h)8.16 (0.99)

8.08 (0.75)

0.544

0.79 (0.54)

0.75 (0.67)

0.153

Social jet lag (h)Values are means and SD
Adjusted by age and sex

Page 5 of 8

Furthermore, in this study, social jet lag tended to increase with high CCT. Social jet lag is known to be larger in night type persons [31, 32]. From the above, thisresult suggests that the use of high CCT at night is associated with the night type chronotype. It has been reported that social jet lag is associated with obesity [29],depressive state [33], academic performance [34], andmenstrual symptoms [35]. Further analysis of the associations between light environments, sleep, and health status is needed.
In the present study, significant differences in sleep
habits between lighting conditions were found in Japanese students but not in Chinese students. One of thereasons for this difference may be the difference in illuminance level. Illuminance level after sunset for the Japanese students was significantly higher than that for the
Chinese students. It is known that the effects of lightalso depend on the illuminance [4]. Therefore, it is possible that the effect of the CCT of light on sleep habits
was small under the condition of the lower illuminancelevel in China. In contrast to light at night, the amountof light exposure in the morning was significantly larger
in the Chinese students than in Japanese students. Thissuggests that the reset of the circadian rhythm by morning light exposure in China might prevent bedtime delay
even though high CCT lighting is used at night.
Furthermore, the effects of light at night on sleep timing may be complicated by other lifestyle factors depending on the country. For example, in this study,
about 80% of the Chinese students lived in dormitories
and about 70% of the Japanese students lived alone.
Chinese students lived with quite regular timetables forclasses. Weak social pressure for a sleep/wake schedule
is considered to be related to the delay of sleep pattern
in Japanese students. This means that Japanese students
are exposed to artificial lighting for a longer period oftime before going to bed, which might be a cause of thedelay of circadian rhythm as shown in a previous study[36]. In other words, relatively strong social pressure forsleep/wake timing in Chinese students might reduce theeffects of light at night on sleep and circadian rhythm.
When Japanese students were divided into those who
lived alone and those who lived with their families, theeffect of CCT was more pronounced among students
who lived alone. This suggests that fewer social constraints for sleep habits make university students more
susceptible to light at night. The relationship between
the type of social constraints and the effect of night lighton sleep needs to be examined in the future.
In this study, more than half of the Japanese and Chinese students used high CCT lighting at night. This result
reflects the fact that it is common to use white and highCCT fluorescent lighting at night at home in some Asian
countries including Japan and China. The percentage of

Higuchi et al. Journal of Physiological Anthropology(2021) 40:7

Page 6 of 8

Fig. 3 Changes in the illuminance level that the participants were exposed to for 24 h in Japan (n = 54) and China (n = 58) (A). The illuminanceon the vertical axis is displayed in logarithm. Average illuminance levels from midnight to early morning (0:00–6:00), in the morning (6:00–12:00),
in the afternoon (12:00–18:00), and in the evening (18:00–24:00) (B). Data are shown as averages and standard deviations. (*p < 0.05, **p < 0.01)

students using high CCT lighting was significantly higher
in Chinese students. The exact reason why white fluorescent lights are often used at home at night in Asia is notknown. White fluorescent lighting began to become popular in Japan after the 1950s, and white fluorescent lighting
becomes the preferred alternative to incandescent light inordinary households. In the 1980s, fluorescent lamps with
low CCT began to be sold, and it is thought that
low CCT lighting has gradually become widespread inJapan. On the other hand, in China, low CCT fluorescentlights are not widely sold in stores and are not widely usedat home. This may be one of the reasons for the differences between Japan and China. Currently, LED lighting
is rapidly becoming widespread in both Japan and China.
Since the CCT of LEDs can be easily adjusted, the lighting
environment at home may be changing. Further researchis needed because the lighting environment can be influenced by culture and technology.
For the survey of CCT light environments, we did
not test its validity since it was a simple question.
When the survey was conducted in 2014–2015, daylight color and warm color fluorescent lamps or incandescent lamps were generally used at home inJapan and China. Today, however, LED lighting with
continuous color adjustment functions is widespread,
and we may therefore need to be more careful about
a survey of CCT of light using a questionnaire.
There were no significant correlations between illuminance level at night and sleep habits in this study.
One reason may be that this study was a field study that
included some uncontrolled factors affecting sleep habits
other than the lighting condition at night. Although the
purpose of this study was not to investigate the cause ofdifferences in sleep habits of Chinese students and Japanese students, it is necessary to consider these factorsthat influence sleep habits. The low reliability of measurements of illuminance level using the pendant sensor
is also a possible cause. For example, the problems withpendant-type sensors are that they may be hidden byclothing and the orientation of the sensor is not stable.
Reliable measurements of illuminance level are needed
to evaluate the relationships between individual sleep
habits and illuminance levels at home. In addition, only
60 of the 447 Japanese and 60 of the 327 Chinese participants had their illuminance measured. Unfortunately,
due to the small sample size in this study, we were also
unable to examine the interaction between CCT and illuminance level.
Furthermore, the relationship between illuminancelevel and sleep habits may be complicated by individual
differences in circadian photosensitivity [37–40]. In ourprevious studies, we found that polymorphisms in themelanopsin gene (OPN4) are associated with both thenon-visual effects of light and the timing of sleep in Japanese university students [40, 41]. It would be interesting
to compare the association between gene polymorphisms and sleep habits in both countries in the future.
In conclusion, the use of blue-enriched white light(high CCT light) at night is associated with delay ofsleep timing in Japanese university students but not in
Chinese university students. Although the effects of lightat night on sleep timing and circadian rhythm may becomplicated by other lifestyle factors depending on the
country, use of low CCT light at home at night is recommended to prevent delay of sleep timing and the circadian rhythm.

Supplementary Information
The online version contains supplementary material available at https://doi.
org/10.1186/s40101-021-00257-x.
Additional file 1: Figure S1. The pictures used in the questionnaire inthis study are not allowed to be shown due to copyright issues, butalmost the same pictures are shown as additional files.
Additional file 2: Table S1. Demographic data and sleep habits inJapanese and Chinese students whose illuminance was measured.

Higuchi et al. Journal of Physiological Anthropology(2021) 40:7

Page 7 of 8

Acknowledgements
We would like to thank Ms. Kaoru Inami for supporting our experiment.

9.

Authors’ contributions
SH, YL, SK, and AY contributed to the design of the experiment. SH, YL, JQ,
and YZ performed the experiments. SH, JQ, YZ, MO, and SL analyzed thedata. SH wrote the manuscript with advice from YL. SK and AY participatedin the discussion and preparation of the manuscript. All authors read andapproved the final manuscript.

10.

11.

Funding
This work was supported by JSPS KAKENHI Grant Number 17K18926.

12.

Availability of data and materials
The datasets analyzed in this study are not publicly available due to a
privacy policy but are available from the corresponding author on
reasonable request.

13.

14.

Declarations
15.

Ethics approval and consent to participate
The study was approved by the Ethics Committee of Kyushu University(Approval No. 157). Written informed consent was obtained from allparticipants.

16.

Consent for publication
Not applicable

17.

Competing interests
The authors declare that they have no competing interests.

18.

Author details
1
Department of Human Science, Faculty of Design, Kyushu University, 4-9-1
Shiobaru, Minamiku, Fukuoka 815-8540, Japan. 2Institute for Electric Light
Sources, Fudan University, Shanghai 200433, China. 3Department of Kansei
Science, Graduate School of Integrated Frontier Sciences, Kyushu University,
4-9-1 Shiobaru, Minamiku, Fukuoka 815-8540, Japan. 4Laboratory ofEnvironmental Ergonomics, Faculty of Engineering, Hokkaido University, Kita
8, Nishi 5, Kita-ku, Sapporo, Hokkaido 060-0808, Japan. 5Department ofSleep-Wake Disorders, National Institute of Mental Health, National Center of
Neurology and Psychiatry, 4-1-1, Ogawa-Higashi, Kodaira, Tokyo 187-8553,
Japan.

19.

20.

21.

22.
Received: 29 December 2020 Accepted: 31 May 2021
23.
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