ORIGINAL PAPER
Nagoya J. Med. Sci. 83. 705–714, 2021
doi:10.18999/nagjms.83.4.705

Perception of feeling cold in the bedroom and sleep quality
Odgerel Chimed-Ochir1, Shintaro Ando2, Shuzo Murakami3,
Tatsuhiko Kubo1, Tomohiro Ishimaru4, Yoshihisa Fujino4
and Toshiharu Ikaga5
2

1
Department of Public Health and Health Policy, Hiroshima University, Hiroshima, Japan
Department of Architecture, Faculty of Environmental Engineering, The University of Kitakyushu,
Kitakyushu, Japan
3
Institute for Building Environment and Energy Conservation, Tokyo, Japan
4
Department of Environmental Epidemiology, Institute of Industrial Ecological Sciences,
University of Occupational and Environmental Health, Japan, Kitakyushu, Japan
5
Department of System Design Engineering, Faculty of Science and Technology, Keio University,
Yokohama, Japan

ABSTRACT
Sleep disorders are drawing the attention of both medical and public health concern worldwide. In
Japan, research suggests that one fifth of adults do not receive appropriate sleep and 40% of adults sleep
less than 6 hours a day, and sleep rates are decreasing further year by year. Many studies show that cold
indoor environments negatively affect sleep comfort and quality. Whereas these studies have focused on
the effects of low bedroom temperature, few studies have focused on the effect of perception of coldness.
Indoor temperature is typically much lower in Japan than in other countries. Therefore, the current study
aimed to identify the effect of perception of bedroom coldness on sleep quality among Japanese adults.
After controlling for covariates of age, presence of current disease and pain, smoking and consumption of
alcohol (Model 1), participants who sometimes, often or always felt cold in the bedroom exhibited 0.57
(95% CI=0.32–0.83, p=<.0001), 1.08 (95% CI=0.82–1.35, p<.0001) or 2.25 (95% CI=1.83–2.67, p<.0001)
higher PSQI scores compared to the group which didn’t feel cold in bedroom. Our findings suggest
keeping the bedroom thermal environment above a minimum limit as recommended by the World HealthOrganization or other organization during colder, winter nights when feeling cold during sleep. Additional
deficiencies in the housing infrastructure, air quality issues due to the use of a heater, and micro bed
environment need to be holistically addressed. Sleep quality can be improved by certain level via providing
thermally comfortable sleeping environment.
Keywords: community health, cold bedroom, indoor thermal environment, housing, sleep quality
Abbreviations:
CI: confidence interval
PSQI: Pittsburgh Sleep Quality Index
MHLW: Ministry of Health, Labor and WelfareThis is an Open Access article distributed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International
License. To view the details of this license, please visit (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Received: November 25, 2020; accepted: February 19, 2021
Corresponding Author: Yoshihisa Fujino, MD, MPH, PhD
Department of Environmental Epidemiology, Institute of Industrial Ecological Sciences, University ofOccupational and Environmental Health, Japan, 1-1 Iseigaoka, Yahata-nishi-ku, Kitakyushu 807-8555, Japan
Tel: +81-93-603-1611, Fax: +81-93-601-7324, E-mail: zenq@med.uoeh-u.ac.jp
705

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Odgerel Chimed-Ochir et al

INTRODUCTION
Japan is unique about its residential indoor thermal environment. Central heating is lesscommon compared to other developed countries, and the indoor temperature is typically lower
in Japan than in other countries.1 In most modern Japanese homes, each room is heated by its
own wall-mounted heating unit. Approximately 39% of existing houses in Japan are uninsulated.
Uniformly heating an entire house or apartment is considered wasteful; it is standard to heat theliving room, but it is not common practice to heat the bedroom. According to a report from the
Ministry of Land, Infrastructure, Transport and Tourism of Japan, the average room temperature
in a Japanese house in winter is 17°C or less, and that of the bedroom is lower than 13°C.2
Research on thermal comfort in the sleeping environment suggests that thermal environmentis one of the primary causes of sleep disturbance among many other factors such as physicalstates, mental states and bedding conditions.3-4 Many studies show that cold indoor environments
negatively affect sleep comfort and quality.4-6 Whereas these studies have focused on the effectsof low bedroom temperature,6 few studies have focused on the effect of perception of coldness.
No previous study has examined the relationship between the perception of cold and sleep
quality in Japan.
Sleep disorders are drawing the attention of both medical and public health concern worldwide.
They have been found to increase the risk of developing various diseases such as cardiovasculardisease7 and depression.8 Sleep has even been linked to mistake rates during exertion and agreater risk of accidents.9 In Japan, research suggests that one fifth of adults do not receive
appropriate sleep,10 40% of adults sleep less than 6 hours a day, and sleep rates are decreasing
further year by year.11 Thus, government and other business entities have engaged with various
sleep initiatives promoting an awareness of sleep disorders. However, little work has been done
examining sleep environment and sleep quality. Therefore, the current study aimed to identify
the effect of perception of bedroom coldness on sleep quality among Japanese adults.

MATERIALS AND METHODS
Subjects
The Ministry of Land, Infrastructure, Transport and Tourism of Japan funded the Smart Wellness Housing project which aimed to quantitatively evaluate the impact of housing condition on
residents’ health between November and March from 2014 through 2017 all over Japan. With
the collaboration of retrofitting companies, we recruited 3,289 participants who were willing
to retrofit their houses and visited to the retrofitting companies for consultation across all 47
prefectures of Japan during November 2015 and March 2016. At each residence, both husband
and wife were invited to answer paper-based questionnaires. All participants are expected tobe re-questioned after retrofitting; however, we only used the data of baseline survey. In thecurrent study, all participants also were questioned about their demographic, socio-economic
status, housing conditions and sleeping quality in accordance with Pittsburgh Sleep Quality Index(PSQI), quality of life in accordance with Short form-8 (SF-8) and GHQ-12, diet, pain, presenceof diseases. In the current study, we only extracted the data of PSQI, information on bedroom
thermal environment and socio-economic factors to suffice the aim of the study.
The study was conducted in accordance with the principles of the Declaration of Helsinki
and approved by the ethical committee of the Hattori Clinic Institutional Review Board. All
participants gave their informed consent for participating in the study. More details of the project
can be found at http://www.umin.ac.jp/ctr/index.htm. (UMIN Clinical Trials Registry, Trial No.

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Feeling cold and sleep quality

UMIN000030601.)
Measurements
Sleep quality.   We used the Japanese version of the PSQI questionnaire which was translated
by Doi et al in 1998 and has been widely used among the Japanese population since then. 12
This is an instrument to measure the quality and patterns of sleep in the older adult. This tool
differentiates “poor” from “good” sleep by measuring seven domains: subjective sleep quality,
sleep latency, sleep duration, habitual sleep efficiency, sleep disturbances, use of sleep medication, and daytime dysfunction over the last month. Mean PSQI values were calculated based onparticipants’ self-assessment of these seven areas of sleep.13 Scoring of the answers is based on
a 0 to 3 scale, whereby 3 reflects the negative extreme on the Likert Scale. A global sum of“5” or greater indicates a “poor” sleeper.
Thermal environment.   The Health Maintenance Promoting Housing Research Council andits consortium, established under the Ministry of Land, Infrastructure, Transport and Tourismof Japan, developed the Comprehensive Assessment System for Built Environment Efficiency(CASBEE) Housing checklist to assess the health of residents within the Smart Wellness Housing project. This checklist has six health elements including thermal environment, acoustic
environment, lighting environment, hygiene, safety and security for 8 distinctive rooms/placesin the home.14 In the current study, we focused on the impact of bedroom thermal environment
including feeling coldness, and a dry nose or throat in the bedroom.
Statistical analysis
The participants were classified into four groups based on their frequency of perceiving thermal
problems in the bedroom (never, sometimes, often and always experiencing problems) to study
the relationship between thermal problems and sleep quality. We also compared sleep quality
between “heater-users in the bedroom” and “non-user” group. Accordingly, descriptive statistics
were calculated with respect to demographic indicators and other variables (sex, gender, education, income etc). Mean PSQI was calculated for each demographic indicator and component ofthermal environment. Linear regression was used to identify the relationship between the perceived
level of thermal problems and PSQI. Linear regression modeling was conducted by adjustingi) without any variables (univariate), ii) age, smoking and alcohol-drinking habits, presence ofdisease and pain, and (Model 1) iii) usage of heater in the bedroom along with variables of ii)(Model 2); and an adjusted parameter estimates were calculated. A Chi square test was carriedout to check association between perception of thermal comfort and usage of heater in bedroom.
Significance was set at the 5% level (p<0.05). SAS Version 9.4 was used to analyze the data.

RESULTSWe sent the questionnaire to 3,289 participants and the response rate was 70 %. We excluded
those who did not answer all areas of the PSQI questionnaire because it is not possible tocalculate the overall PSQI if one area is missing. In addition, participants who failed to answer
questions on thermal comfort or heater usage were excluded from the analysis. Upon exclusion,a total of 2,193 participants were analyzed in the current study.
The demographic characteristics of the study participants are presented in Table 1. The mean
age was 52.6±14.1; 47.4% were men and 52.6% were women. The majority of participants were
either high school (38.7%) or technical college graduates (26.0%) and 29.5% obtained either a708
Odgerel Chimed-Ochir et albachelor’s or higher degree. With regard to income, the majority of participants (73.4%) earned
from 2 to 9 million Japanese Yen (approx. 19,000–85,000 USD) per year. More than 80% ofparticipants were married. The mean PSQI score was 4.96±2.54 among all participants. The highest (5.70±3.02) and lowest (4.48±1.97) PSQI score were attributed to divorced/widowed people
and those who had the highest annual income, respectively. More than 90% of participants were
either current or past smokers and almost one-third (27.1%) reported to drink alcohol every day.
Half (49.8%) and almost half (40.5%) of participants had pain and any type of diseases during
the study period, respectively.

Table 1

Demographic characteristics of study participants and mean PSQI

Demographic Indicators
Age1)
Sex2)
Male
FemaleEducation2)
Junior-high school
High school
Technical college
University or over
Income (annual)2)
Up to 2 million JPY
2–5 million JPY
5–9 million JPY
Over 9 million JPY
Marital status2)
MarriedSingleDivorced/Widowed
Smoking status3)
Current smoker
Past smoker
Never smoked before
Alcohol consumption
EverydaySometimes
Not drinking
Presence of pain
Yes
No
Presence of diseases
YesNo# (or Mean)
52.6% (or SD)
(14.1)

Mean PSQI
4.96

SD(2.54)

1039
1154(47.4)(52.6)

4.77
5.13(2.43)(2.63)

122
819
550
624

(5.8)
(38.7)(26.0)(29.5)

5.31
4.95
5.06
4.80

(3.15)(2.55)
(2.5)
(2.4)

95
403
328
170

(9.5)
(40.5)(32.9)(17.1)

5.52
5.00
4.70
4.48

(3.19)(2.46)(2.33)(1.97)

1790
244
145

(82.1)(11.2)
(6.7)

4.89
5.03
5.70

(2.47)(2.66)(3.02)

339
1562
164

(16.4)(75.7)
(7.9)

4.90
4.92
5.08

(2.55)(2.5)(2.68)

589
640
947

(27.1)(29.4)(43.5)

4.73
4.94
5.10

(2.39)(2.39)(2.69)

1082
1091(49.8)(50.2)

5.37
4.55(2.65)(2.36)

626
918(40.5)(59.5)

5.30
4.62(2.74)(2.28)

Shown as mean and SDShown as number and percentage3)
Current smoker: Had smoked 100 or more cigarettes during entire life or had smoked during last
6 months
1)
2)

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Feeling cold and sleep quality

The descriptive statistics for estimated PSQI in relation to the bedroom thermal environment
are in Table 2. The majority of participants reported feeling cold (66.9%) and dry nose andthroat (81.3%) in the bedroom. More than 70% of participants didn’t use any type of heater inthe bedroom. Those who felt cold and experienced a dry nose and throat more frequently inthe bedroom showed higher PSQI score.
Also presented in Table 2 is the relationship between PSQI and perception of bedroom thermal
environment and heater usage. We ran three separate models as described in the Methods section.
By the univariate model, participants who sometimes, often or always felt cold in the bedroom
exhibited 0.61 (95% CI=0.36–0.86, p=<.0001), 1.14 (95% CI=0.88–1.41, p<.0001) or 2.38 (95%CI=1.96–2.79, p<.0001) higher PSQI scores compared to the group which didn’t feel cold in
bedroom. After controlling for covariates of age, presence of current disease and pain, smoking
and consumption of alcohol (Model 1), participants who sometimes, often or always felt cold in
the bedroom exhibited 0.57 (95% CI=0.32–0.83, p=<.0001), 1.08 (95% CI=0.82–1.35, p<.0001)
or 2.25 (95% CI=1.83–2.67, p<.0001) higher PSQI scores compared to the group which didn’t
feel cold in bedroom.
Compared to those who didn’t use a heater in bedroom, those who used a heater had significantly higher PSQI scores. This result pattern was consistent for Model 1 and Model 2 (Table 2).

Table 2

Descriptive statistics of PSQI by thermal comfort and relation between PSQI and thermal comfort

Subjects
PSQI
n (%)
M (SD)
Feeling coldness in bedroom
Not at all
726 (33.1) 4.3 (2.3)
Sometimes
725 (33.1) 4.9 (2.4)
Often
579 (26.4) 5.4 (2.6)
Always163 (7.4)
6.7 (2.9)
Feeling dry nose and throat in bedroom
Not at all
410 (18.7) 4.2 (2.4)
Sometimes
521 (23.8) 4.7 (2.5)
Often
918 (41.9) 5.1 (2.4)
Always344 (15.7) 5.9 (2.7)
Using heater in bedroom
Yes
1571 (71.8) 4.9 (2.5)
No
622 (28.2) 5.2 (2.6)

Univariate
PE (95% CI)

Model 1 1)
PE (95% CI)

Model 2 2)
PE (95% CI)

Reference
0.61 (0.36–0.86)*** 0.57 (0.32–0.83)*** 0.57 (0.32–0.82)***
1.14 (0.36–0.86)*** 1.08 (0.82–1.35)*** 1.07 (0.80–1.34)***2.38 (1.96–2.79)*** 2.25 (1.83–2.67)*** 2.23 (1.81–2.65)***
Reference
0.46 (0.14–0.78)** 0.40 (0.09–0.72)* 0.40 (0.08–0.72)*
0.95 (0.66–1.24)*** 0.90 (0.60–1.19)*** 0.89 (0.60–1.18)***1.68 (1.32–2.03)*** 1.60 (1.24–1.97)*** 1.59 (1.22–1.95)***
Reference
0.34 (0.10–0.57)**

0.29 (0.06–0.52)**NA***p<0.001, **p<0.01, *p <0.05
PSQI: Pittsburgh Sleep Quality Index
M: mean
SD: standard deviation
PE: parameter estimates
CI: confidence interval
1)
Adjusted with presence of pain, disease, age, smoking, and alcohol.
2)
Further adjusted with using heater in the bedroom.

Table 3 shows the association between perception of thermal comfort and heater usage in the
bedroom. A significant association was found between heater usage and perception of coldness

710
Odgerel Chimed-Ochir et al(p=0.0257) and throat and nose dryness (p=0.0388) in the bedroom. By looking at cell chi-square,those who use a heater in bedroom tend not to feel cold but always feel a dry nose and throat.

Table 3 Association between perception of thermal comfort and usage of heater

Using heater in bedroom
Frequency %Cell χ2
Feeling coldness in bedroom
Not at all
182
8.3
2.78
Sometimes
202
9.21
0.06
Often
183
8.34
2.15
Always55
2.51
1.66
Feeling dry nose and throat in bedroom
Not at all
104
4.74
1.30
Sometimes
137
6.25
0.79
Often
264
12.04
0.05
Always
117
5.34
3.87

Not using heater in bedroom
Frequency %Cell χ2

P value

544
523
396
108

24.81
23.85
18.06
4.92

1.10
0.03
0.85
0.66

0.0257

306
384
654
227

13.95
17.51
29.82
10.35

0.51
0.31
0.02
1.53

0.0388DISCUSSION
Our study on the impact of perception of bedroom thermal environment on sleep quality foundout that compared to those who didn’t feel cold in the bedroom, participants who felt cold hadsignificantly higher PSQI scores indicating poor sleep quality. The result was very consistent for
both non-adjusted and adjusted models which may reflect that people’s perception of coldness isa significant contributor to sleep quality regardless of their demographic background and heater
usage.
Several researchers and organizations recommend a minimum air temperature in the bedroom.
For example, the World Health Organization recommends a minimum air temperature of 18°C
for bedrooms15 and the European standard16 recommends a minimum temperature of 20°C for
heating in living spaces including bedrooms. Pan et al predicted the thermal neutral temperature
to be 18.5°C, but the subjects had better sleep quality at a much higher temperature of 23°C
in winter.17 Some people feel comfortable sleeping in a warmer environment while others prefera cooler bedroom because thermal sensation varies person to person, and therefore affect a
subject’s whole-body thermal comfort and sleep quality.18 Consequently, it is quite complicated todefine the ideal bedroom temperature for good quality of sleep and no widely accepted explicit
bedroom temperature has been established.19 Bedroom thermal sensation and ideal temperature
is likely to be further complicated due to differences in local climates, use of bedding systems
and sleeping habits worldwide.20,21
Thus, we strongly recommend that a bedroom temperature should be properly controlled to
one’s own comfort along with improving the bed microclimate to avoid feeling cold either at
bedtime or during sleep regardless of recommended room temperature.
To date, whereas several studies have examined the impact of low bedroom temperature on
sleep quality,6 research on the perception of cold and its relationship with sleep quality is limited.
Research on bedroom thermal environment reports that sleeping in cold environments significantly
affects the sleep quality in naked subjects, although this negative effect could be reduced by the711
Feeling cold and sleep quality

use of bed coverings and clothing.22,23 In reality, people don’t sleep naked and without bed coverings, especially in a cold bedroom. Several other studies confirm that sleeping within a range of
13–20°C will not affect sleep quality when a blanket is used.4 Similarly, without increasing room
temperature, a thermally comfortable sleeping environment can be created and sleep quality canbe maintained in cold bedrooms24,25 by improving micro-bed factors such as mattress, covering,
sleep wear and percentage of coverage, and the total insulation of the bedding system.26
While there are extensive studies emphasizing that sleep quality may be maintained in acold bedroom, we should not ignore the available evidence from laboratory and clinical studies
on the negative effects of cold air inhalation. Cold air can cause pathophysiological responses(eg, vasoconstriction in the respiratory tract mucosa and suppression of immune responses) that
increase the susceptibility to infections.27 Cold air inhalation was found to significantly affectthe autonomic cardiac response without affecting sleep quality.4 Thus, the entire sleep system,
which includes the bedding ensemble and sleepwear, should be considered holistically alongwith the ambient conditions.
In Japan, it is standard practice to heat only the living room, rather than heating the entire
apartment or house, as seen in Europe and America. It is not common to heat the bedroom inJapan. As a result, average bedroom temperatures are much lower in Japan. A nationwide survey
in 2014 evaluating the winter heating situation reported that bedroom temperatures are lower
than 14.4 throughout all prefectures in Japan except Hokkaido, where central heating systems arecommon, and Okinawa, which is the warmest prefecture in Japan.28 “Warming up at bedtime”
in the Japanese culture means to sleep wearing warm nightwear and covering up with a thickblanket. This behavior would seem to support sleep quality under cold conditions. Indeed, about
50% of elderly Japanese use two to six layers of sleepwear and three to five layers of bed
covers.29 Wearing too many layers in bed may also disrupt sleep comfort. This, again, suggeststhat a holistic approach is needed to maintain good and comfortable sleep by considering the
micro bed climate and sleepwear along with the bedroom temperature.
The cold indoor temperatures are often a consequence of a dwelling’s structural deficiencies,
including a lack of insulation and airtightness, and lack of heating. In addition, countries withmild winters often have homes characterized by poor domestic thermal efficiency that are harderto heat than well insulated houses in more extreme climates.15 This is the case in Japan. As
of 2017, 39% of all Japanese houses were estimated to be uninsulated.1 Therefore, retrofitted
insulation may help to improve mitigate the effect of cold housing in Japan.
Our current study showed that heater use in the bedroom was associated with poorer sleep.
We predicted the opposite because we believed that people are unlikely to feel cold when aheater is used and thus have better sleep quality. We suggest two possible explanations. First,
Japanese people may only turn on a heater when they feel cold, and not constantly to keep the
bedroom warm. Therefore, study participants might have been disturbed by coldness during thenight, which made people wake up and turn on a heater. Second, people who use a heater in the
bedroom usually set their bedroom temperature based on their pre-sleep state. Thus, participants
who felt cold before sleeping could have set comparatively higher temperature which in turndisrupted the sleep during later stage of sleep. Therefore, in case of studying the effect of heatedroom on sleep quality, it is suggested to set a standard bedroom temperature, as recommended
by the World Health Organization or other organization before sleeping.
Our study showed that those who felt dry nose and throat in the bedroom experienced poorer
sleep quality. Supplying fresh air to the head and face of sleeping people could be used as apotential ventilation principle for sleeping people. However, bedrooms are poorly ventilated in
winter time because opening a window makes the room much colder. In addition, some type ofheaters creates carbon emission and almost all types of heaters pull moisture from the air.30 Long

712
Odgerel Chimed-Ochir et aluse of forced air heating systems, and both electric and gas heaters cause substantial dryness
and deters people from using these kinds of heaters. Our study showed that those who use aheater in the bedroom tend not to feel cold but frequently experience a dry nose and throat.
This may deserve more studies on a holistic approach of cost efficient and health-friendly heating
systems along with good ventilation. Thus, sleep quality can be improved by certain increment
via providing thermally comfortable sleeping environment.
The current study has two notable strengths. First, more than half of family involved in thecurrent study had annual income over 4 million Japanese Yen (approximately 38,000 USD)
which is higher than median annual family income in Japan.31 In addition, all families includedin the study were those who contacted to retrofitting companies for their renovation of theirhouses and were able to pay the expenses, therefore, the study population is likely to be quite
homogeneous in terms of their socioeconomic status, which should have reduced the confounding
effects related to socioeconomic factors (eg, poverty level). Second, this survey covered all 47
prefectures of Japan, thus, sampling bias is significantly reduced in terms of regional variation.
Our assessment is all subjective. In the present study, we did not measure the participants’room temperature, but rather questioned their perception of feeling. This could be considereda limitation of the current study. We were unable to examine whether increasing or decreasing
room temperature could change the sleep quality, nor can we offer a recommended temperaturethat appears to support good health.

CONCLUSION
The current study adds important information to the growing body of evidence that proper
thermal environment contributes to people’s quality of sleep. Our findings suggest keeping the
bedroom thermal environment above a minimum limit as recommended by the World HealthOrganization or other organization during colder, winter nights when feeling cold during sleep.
However, consistent heating in the bedroom may not be a sufficient measurement. Additional
deficiencies in the housing infrastructure, air quality issues due to the use of a heater, and micro
bed environment need to be holistically addressed. Sleep quality can be improved by certain
level via providing thermally comfortable sleeping environment.

ACKNOWLEDGMENTS
This study was partly supported by the Ministry of Land, Infrastructure, Transport and Tourism
as part of the Model Project for Promotion of SWH and a JSPS KAKENHI (Grant Numbers
JP17H06151)

CONFLICT OF INTEREST
T.Ikaga. has received research grants (significant) from Tokyo Gas Co., Ltd., Osaka Gas Co.,
Ltd., HyAS & Co. Inc., Fuyo Home Co. Ltd., Asahi Kasei Homes Corp., OM Solar Co. Inc.,
Kajima Corp., Shimizu Corp., Nice Corp., Japan Gas Association and Japan Sustainable Building
Consortium. M.S. has received nonrestrictive research funds (significant) from Taiyo Nippon Sanso
Corp. Y.F. reports grants and personal fees from Saibugas Co.,Ltd. outside the submitted work.

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Feeling cold and sleep quality

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