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Abstract In order to examine whether the spectral
compositions of light source may affect sleep quality, sleep
architecture under different color temperatures of light sources
was evaluated. Seven healthy males were exposed to the light
sources of different color temperatures (3000 K, 5000 K and
6700 K) for 6.5 h before sleep. The horizontal illuminance
level was kept at 1000 lux. Subjects slept on a bed in near
darkness (10 lux) after extinguishing the light, and
polysomnograms recorded the sleep parameters. In the early
phase of the sleep period, the amount of stage-4 sleep (S4-
sleep) was significantly attenuated under the higher color
temperature of 6700 K compared with the lower color
temperature of 3000 K. Present findings suggest that light
sources with higher color temperatures may affect sleep quality
in a view that S4-sleep period is important for sleep quality.
J Physiol Anthropol Appl Human Sci 24 (2): 183–186, 2005
http://www.jstage.jst.go.jp/browse/jpa
[DOI: 10.2114/jpa.24.183]
Keyword: color temperature, slow-wave sleep, polysomnogram,
fluorescent light
Introduction
In contemporary society, Japanese people tend to suffer
from inadequate sleep; the prevalence rates of sleep problems
have been estimated as 20.9–44.3% in the general Japanese
adult population (Doi et al., 2001). Thus, environments that
add comfort to favorably promote sleep are required.
It has been found that light suppresses nocturnal secretion of
melatonin (Lewy et al., 1980; Hasihimoto et al., 1996).
According to Morita and Tokura (1996), light sources with
higher color temperatures suppress the nocturnal secretion of
melatonin compared with those of lower color temperatures.
Melatonin has been thought to have a sleep-promoting
function (Zhdanova et al., 1996) and plays an important role in
the regulation of sleep quality (for review see Dawson and
Encel, 1993).
However, Deguchi and Sato (1992) have observed that
greater amplitudes of contingent negative variation (CNV) are
generated under conditions with a higher color temperature.
This finding suggests that a light source with higher color
temperatures would more likely activate the reticular activating
system (RAS) compared with that of lower color temperatures.
In comparisons of the autonomic nervous functions (ANF),
Mukae and Sato (1992) evaluated heart rate variability (HRV)
under conditions of different color temperatures, and found
that enhancements of parasympathetic and sympathetic nerve
functions were established with light sources of higher color
temperatures, concluding that light illumination of higher color
temperatures activates ANF more than that of lower color
temperatures. Furthermore, our previous study has demonstrated
the activation of sympathetic nerve function during night-time
exposure to a light source of higher color temperatures
(Tsutshumi et al., 2002).
In addition to the reduction of nocturnal melatonin secretion
and activation of the ANF and RAS, light sources of higher
color temperatures would affect sleep quality. In this study,
we compared sleep architectures after exposures to light
source of different color temperatures, and evaluated the
effects of spectral compositions of light sources on sleep
quality.
Methods
Subjects
Seven young male adults (meanS.D.212.1 years) gave
informed consent and participated in this experiment. All
subjects were physically normal and healthy. They were asked
to wear thin sleeveless shirts and shorts during the experiment,
and not to take a nap before the experiment.
Experimental design
This study was carried out from July to November. Subjects
participated for 4 nights in the study according to the
experimental regimen (Fig. 1). The first night was defined as
the “adaptation night”, and exposures to the 3 color
Effect of Color Temperature of Light Sources on Slow-wave Sleep
Tomoaki Kozaki
1)
, Shingo Kitamura
2)
, Yuichi Higashihara
2)
, Keita Ishibashi
1)
,
Hiroki Noguchi
3)
and Akira Yasukouchi
1)
1) Department of Physiological Anthropology, Faculty of Design, Kyushu University
2) Department of Ergonomics, Kyushu Institute of Design
3) Matsushita Electric Works, Ltd.
Journal of
PHYSIOLOGICAL
ANTHROPOLOGY
and Applied Human Science
temperatures (3000 K, 5000 K and 6700 K) were conducted on
3 different nights at a rate of 1 color temperature per night.
Three experimental conditions with different color
temperatures were performed in a random order. Data from the
adaptation period were excluded from the analysis because
subjects indicated longer awake periods and less rapid eye
movement (REM) sleep (Agnew et al., 1966). The physical
characteristics of light sources are listed in Table 1.
Subjects entered an experimental chamber at 18 : 00 h. They
were exposed to control lighting for 1 h, and allowed to bathe
at 19 : 00 h. The horizontal illuminance level of control lighting
of 10 lux was designated at a reference level (height from the
center of the chamber floor: 90 cm). After bathing, subjects
were exposed to a light stimulus (3000 K, 5000 K or 6700 K)
from 19 : 30 h to 2 : 00 h, and had supper at 20 : 30 h. The
supper consisted of a routine meal. The subjects were
instructed to rest on a sofa and to remain awake during the
light exposure. They were allowed to listen to music and given
access to reading materials. Light for each color temperature at
a horizontal illuminance level of 1000 lux was adjusted at the
reference level. The subjects slept on a bed in near darkness
(10 lux) from 2 : 00 h to 9 : 00 h. The ambient temperature in
the experimental chamber was kept at 25°C with 50% relative
humidity.
Recording and scoring of sleep records
Electroencephalography (EEG), submental electromyography
(EMG) and electrooculography (EOG) were recorded at
10 mm/sec by electroencephalography (EEG-5214, NIHON
KOHDEN Co. Ltd., Japan). For EEG recording, the Ag/AgCl
electrodes were attached to two scalp sites (C3 and C4) and
both earlobes (A1 and A2) according to the International 10/20
system. Polysomnograms were scored in 30-s epochs
according to international criteria (Rechtschaffen and Kales,
1968).
Data analysis
Stage-4 sleep (S4-sleep) is known to occur predominantly
during the early part whereas REM sleep increases during the
later part of the sleep period (Williams et al., 1964). Thus, the
sleep period (excluding sleep latency) were divided into the
early (P1; 02 : 00–05 : 30 h) and the late (P2; 05 : 30–09 : 00 h)
phases in the present study. Using P1 and P2 of the sleep
period, color temperature and subject as the variables of sleep
architectures, correlations of these variables were analyzed by
the three-way analysis of variance (ANOVA). Sleep latency
(SL) was analyzed using the two-way ANOVA with subject
and color temperature as the variables. The multiple
comparison test (Turkey’s HSD) was used for subsequent
analysis. Differences where p0.05 were considered
statistically significant.
Results
Based on the meanstandard deviations of SL (Table 2),
significant differences of SL were not established under
conditions of different color temperatures [F(2,12)0.14,
p0.1].
There were significant effects on the phase of sleep periods
for REM [F(1,6)63.03, p0.01], S2- [F(1,6)10.65,
p0.05], S3- [F(1,6)10.09, p0.05] and S4- [F(1,6)26.9,
p0.01] sleep (Table 3). Most of REM sleep was obtained in
184 Effect of Color Temperature of Light Sources on Slow-wave Sleep
Fig. 1 Experimental design. Subjects were exposed to the respective light stimuli (3000 K, 5000 K, 6700 K) at one color temperature per night
from 19 : 00–2 : 00 h before sleep, bathed at 19 : 00 h and took supper at 20 : 30 h.
Table 1 Physical characteristics of light sources
Lighting source Type Model number Ra x-axis
y-axis
Actual color temperature*
Control incandescent lamp LDS100V38W WK 100 0.463 0.415 2700 K
3000 K fluorescent lamp FHF32EX-L-H 84 0.438 0.392 2900 K
5000 K fluorescent lamp FHF33EX-N-H 84 0.349 0.349 4900 K
6700 K fluorescent lamp FHF34EX-D-H 84 0.316 0.319 6500 K
Chromaticity coordinate (CIE: Commission Internationale de L’Eclairage, 1931)
* Actual expression was correlated with the color temperature
All light sources were manufactured by Matsushita Electric Industrial Co. Ltd. Japan
Table 2 Sleep latencies (SL) under different color temperatures are ex-
pressed as the meanstandard deviations (min). No significant differ-
ences between any 2 of the 3 color temperatures at any one time were
derived
3000 K 5000 K 6700 K
SL 6.93.1 4.94.1 6.512.4
P2 of the sleep period, as well as S2-sleep. However, the
amount of S3- and S4-sleep increased in P1 of the sleep
period.
Result analysis by ANOVA for S4-sleep indicated the
tendency of color temperature effects [F(2,12)3.79,
p0.052] and an interactive effect between P1 or P2 of sleep
period and the color temperature [F(2,12)3.32, p0.071].
From the sleep variables for each color-temperature condition
in P1 of the sleep period (Table 4), the amount of S4-sleep was
reduced under conditions with 6700 K compared with 3000 K
lighting (p0.05). In P2 of the sleep period (Table 5), however,
there were no significant differences correlating the sleep
variables with the different color temperatures.
Discussion
Based on the characteristic changes in sleep patterns
(Williams et al., 1964), our data showed higher amounts of S3-
and S4-sleep in P1 than in P2 of the sleep period (Table 3). In
P1 of the sleep period (Table 4), significantly less amount of
S4-sleep was obtained under 6700 K than under 3000 K
lighting. It is considered that slow wave sleep (SWS; S3- and
S4-sleep) may be important for the enhancement of sleep
quality. Webb and Agnew (1970) have compared the sleep
architectures in subjects having different lengths of the sleep
period. They found no significant differences in S4-sleep
between the short and long sleepers, although the former
manifested less REM sleep than the latter. These findings were
interpreted to indicate that the short sleepers spent less time in
light-sleep and awakenings. Kecklund and Åkerstedt (2004)
have demonstrated that mental stress may have a negative
effect on SWS. Subjects who have high apprehensions towards
the next working day show abbreviated SWS periods and lower
scores in subjective sleep quality. Furthermore, SWS in
depressed patients has been documented to have a positive
correlation with the subjective estimation of sleep duration
(Rotenberg et al., 2000). Given that the S4-sleep period is
important for sleep quality, our findings suggest that light
sources of higher color temperatures may reduce sleep quality
compared with those of lower color temperatures.
Previous studies have demonstrated the effects of light
sources with different color temperatures on HRV (Mukae and
Sato, 1992; Tsutsumi et al., 2002), blood pressure (Kobayashi
and Sato, 1992), EEG (Küller and Wetterberg, 1993) and CNV
(Deguchi and Sato, 1992) in humans. These findings imply that
the effects of pre-sleep exposures to certain light sources may
affect sleep quality. However, changes in the nocturnal
secretion of melatonin manifested in our results were likely
caused by the different conditions with lighting of different
color temperatures. Van Den Heuvel et al. (1997) have
reported that the administration of atenolol, a
b
-blocker,
decreases the amount of SWS period compared with placebo
administration;
b
-blockers have been known to alter normal
melatonin production. In addition, decreases in SWS are
reversed with melatonin treatment after atenolol administration
in their investigation. The spectral region between 446 and 477
nm has been regarded as lighting with the most potent
wavelength for regulating melatonin secretion in humans
(Brainard et al., 2001). The light source of 6700 K includes the
action spectrum for suppression melatonin release compared to
that of 3000 K, and might decrease the amount of S4-sleep in
tandem with the changes in melatonin secretion.
In this study, relatively high light illuminance at 1000-lux
was used. A previous study has demonstrated suppression of
melatonin secretion when the human retina is exposed to100-
Kozaki, T et al. J Physiol Anthropol Appl Human Sci, 24: 183–186, 2005 185
Table 3 Sleep architectures of the early (P1) and late (P2) phases of
sleep period. Values (min) are represented as the meanstandard
deviations. Differences where p0.05 or 0.01 were considered
significant on comparisons
Early phase Late phase
WASO 2.55.7 1.43.2
REM 26.212.9 55.719.4**
S1 18.613.1 24.712.8
S2 100.721.6 113.721.8*
S3 30.114.2 11.18.6*
S4 27.814.7 4.24.8**
WASO, wake after sleep onset
** p0.01; * p0.05
Table 4 Sleep architectures for different color temperatures in the early
phase (P1) of sleep period. Values (min) are expressed on the mean
standard deviations. Differences where p0.05 were considered
significant on comparisons
3000 K 5000 K 6700K
WASO 1.31.6 1.43.4 4.89.2
REM 31.110.4 23.013.5 24.515.1
S1 15.06.7 17.97.4 22.920.9
S2 92.726.3 107.021.1 102.317.0
S3 29.415.1 30.616.9 30.112.5
S4 34.917.6 25.98.9 22.415.2*
WASO, wake after sleep onset
*p0.05, 3000 K vs. 6700 K
Table 5 Sleep architectures with different color temperatures in the late
phase (P2) of sleep period. Values (min) are expressed on the mean
standard deviation. No significant differences in all parameters were
indicated on comparison of any 2 of the 3 color temperatures at any
one time
3000 K 5000 K 6700 K
WASO 0.60.5 2.25.4 1.41.4
REM 56.120.7 60.619.5 50.319.4
S1 26.214.4 21.911.9 25.913.7
S2 117.525.3 111.825.4 111.716.5
S3 9.06.8 11.49.7 12.910.0
S4 3.43.5 2.63.5 6.66.6
WASO, wake after sleep onset
lux white light illuminance (Glickman et al., 2003). This
finding suggests that a light source with different color
temperatures may affect sleep quality in the home. In other
words, the use of an appropriate light source may improve the
quality of our living environment.
Acknowledgement This research was supported in part by
a Grant-in-Aid for Scientific Research (S: No. 15107006) from
Japan Society for the Promotion of Science (JSPS) and a
Grant-in-Aid for the 21
st
Century COE Program from the
Ministry of Education, Culture, Sports, Science and
Technology (MEXT).
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Received: July 21, 2004
Accepted: February 7, 2005
Correspondence to: Tomoaki Kozaki, Department of Physio-
logical Anthropology, Faculty of Design, Kyushu University,
4–9–1 Shiobaru, Minami-ku, Fukuoka 815–8540, Japan
Phone: 81–92–553–4530
Fax: 81–92–553–4530
e-mail: kozaki@design.kyushu-u.ac.jp
186 Effect of Color Temperature of Light Sources on Slow-wave Sleep