Okamoto‑Mizuno and Mizuno ﻿
Journal of Physiological Anthropology(2024) 43:2
https://doi.org/10.1186/s40101-023-00350-3

ORIGINAL ARTICLE

Journal of
Physiological Anthropology

Open AccessHow staying in a gymnasium affects sleepand bed climate in children
Kazue Okamoto‑Mizuno1*   and Koh Mizuno2

Abstract
Background We investigated the relationship between sleep, ambient climate, and bed climate in school-aged
children during a one-night stay in a simulated shelter in a gymnasium to demonstrate the effect of ambient climate,and bed climate on sleep.
Methods We obtained measurements during a one-night stay in a school gymnasium (C), days before C (BC),on the day after (A1), and on the second day after C (A2) in 13 healthy school-aged children during summer. Sleep
was evaluated using an actigraph, and the temperature and humidity of the bedrooms in the participants’ homes
and in the gymnasium were monitored for 3 days before and after C. The bed climate of the chest and foot areas
was measured for two nights before and after C. The participants were asked to report on their subjective sleep esti‑
mations and thermal sensations two nights before and after C.
Results The ambient temperature in C was significantly higher than that in BC at the initial 180 min, while it signifi‑
cantly decreased compared to A1 and A2 in the last 100 min. The ambient humidity was significantly higher in both C
and BC compared to A1 and A2. The sleep efficiency index decreased significantly in C (43±4.8%) compared to otherconditions. Sleep time was significantly shorter in C than in other conditions during the initial 195 min. The increasein the bed climate temperature of the chest area in C was significantly delayed compared to that in the other condi‑
tions, around the initial 30 min after the lights were off. Subjective estimation revealed that in C, 85% of the partici‑
pants were out of their sleeping bag at sleep onset, and their sleep was disturbed by heat (77%).
Conclusions Our study revealed that the disturbed sleep patterns observed with children in a simulated shelter may
be related to a delayed increase in bed climate temperature in the chest area. This delayed increase could be related
to the children not entering the sleeping bag and a delayed chest skin temperature increase during the sleep onset
period.
Keywords Bed climate, Sleep, Simulated shelter, Gymnasium, Children*Correspondence:
Kazue Okamoto‑Mizuno
k-mizuno@wayo.ac.jp
1
Faculty of Human Ecology, Wayo Women’s University, 2‑3‑1 Konodai,
Ichikawa, Chiba 272‑8533, Japan
2
Faculty of Education, Tohoku Fukushi University, 6‑149‑1 Kunimigaoka,
Aoba, Sendai, Miyagi 989‑3201, Japan

BackgroundIn the case of impactful disasters such as the Great East
Japan Earthquake of 2011, many shelters were establishedin a park and in school gymnasiums [1]. Previous studies
indicate that staying in shelters after a disaster is accompanied by sleep disturbances due to various factors. In theelderly who evacuated to a school gymnasium during theGreat East Japan Earthquake, severe sleep disturbanceswere identified using actigraphic sleep measures. These
disturbances were due to cold sensations from the floor,
anxiety, and aftershocks [2]. In a survey of 250 evacuees© The Author(s) 2023. 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 appropriate credit to theoriginal author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images orother third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line
to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory
regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of thislicence, visit http://creativecommons.org/licenses/by/4.0/. The Creative Commons Public Domain Dedication waiver (http://creativecom‑mons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated in a credit line to the data.

Okamoto‑Mizuno and Mizuno ﻿Journal of Physiological Anthropology

who stayed in a gymnasium for 1.5 months after theGreat East Japan Earthquake, the most frequently notedreasons for sleep disturbances were noise, a sensation ofcold coming from the floor, and lighting [3]. In this survey, a cold sensation from the floor was claimed despite
an ambient temperature of 20°C. In our previous study
conducted in the summertime, during a one-night stay ina school gymnasium with 109 people, children slept for ashorter period of time, while adults experienced no effect[4]. Sleep disturbance factors in children included noise,lighting, uncomfortable sleeping bags, and heat, while thestiffness of the floor, uncomfortable pillows, noise, andcold were factors for adults. In our another prior study inthe summertime, children’s sleep was severely disturbed
during a one-night stay in a gymnasium, mainly due toheat, even at an ambient temperature of around 26°C [5].
These previous studies suggest that environmental factors, especially thermal discomfort, are among the mostimportant elements affecting sleep in shelters. Thermal
discomfort during sleep is related to both ambient climate and bedding conditions because most shelters inJapan lack air conditioning, sufficient space, adequate
bedding, and portable beds, which cause evacuees tosleep directly on the solid floor.
The thermal environment is one of the most criticalenvironmental issues involving sleep. The temperatureand humidity of the microclimate between humans andclothing (microclimate) and bed covers (bed climate)also play crucial roles in creating a warm and dry bed climate to support sleep [6]. The bed climate temperatureand relative humidity (RH) in the waist area are generally
maintained around 32–34°C, 40–60% RH during normal
sleep [7, 8], which is in accordance with the range of comfort bed climate [9]. Furthermore, bed climate humidity higher than 70% is related to sleep disturbance underhigh ambient temperatures in adults [9]. In school-aged
children, the bed climate temperature of the foot area insummer and fall is maintained at 28–30°C, 50–70% RH
when sleep is in the normal range [10]. However, to thebest of our knowledge, it is still unknown whether the
bed climate of school-aged children is related to sleep.
Since sleep disturbance factors in children staying in the
gymnasium include hot environments and uncomfortable sleeping bags, it is of great interest to investigate therelationship between sleep and bed climate in children
sleeping in a shelter. In our previous study, ambient climate and sleep were measured, and subjective assessments were asked to 20 school-aged children during a
one-night stay in the gymnasium [5]. In the same study,
we measured bed climate in 15 out of these 20 children.
However, since the previous study focused on various
ambient environment factors and sleep, including naps,
the results of bed climate as well as the time course of(2024) 43:2

Page 2 of 12

ambient climate were not considered. We aimed to investigate the relationship among sleep, ambient climate, andbed climate in school-aged children staying in a simulated shelter in a school gymnasium. This information
may shed light on the thermal environmental factors that
affect sleep in a shelter and contribute to practical applications for disaster prevention in the future.

Material and methods
Participants

The methods for screening the participants and thedetails of the participants were the same as those in ourprevious study [4, 5, 11]. Among 20 participants in ourprevious study, 15 participants who were in the fifth andsixth grades in Japanese elementary school were selected
for bed climate measurements together with sleep
parameters and subjective assessments. We excluded twoparticipants from the final analysis because of the termination of bed climate measurements due to body movements. Finally, we analyzed 13 (6 males and 7 females)
participants. The means and standard deviations (SDs)of the children’s age and physical characteristics were
11.4±0.7 years (age), 152.2±4.9 cm (height), 32.5±7.0 kg(weight), and a Lohrel index of 109.6±9.2. The study’s
protocol was informed to the participants and their parents, prior to giving their written consent. This study wasapproved by the Ethics Committee of Tohoku Fukushi
University.
Procedure

We performed the study during a one-night stay in aschool gymnasium in summertime; the event was organized at an elementary school (summer camp). In total,
174 people slept in the gymnasium (123 children, 45 parents, and six teachers).
The objectives and details of the summer camp
have been described in a previous study of ours [5].
The participants were asked to wear a wrist actigraph(Micromini, AMI) on their non-dominant wrist for 7
consecutive days; from 3 days prior to 3 days after theschool camp. Measurements were then recorded from
Wednesday to Tuesday. The children attended school
until 12:30 on Saturdays, and Sundays and Mondays
were holidays. The actigraphs were removed when children attended school on weekdays. Their mothers wereasked to record information in a sleep log, as well as
instances in which their children removed their actigraph. During the activity monitoring period, we continuously measured the temperature, relative humidity(RH), and lighting in the bedroom at 2-min intervals
using a thermistor, hygrometer, and illumination meter(RS-12, Espec Mic Corp). At the school camp, we continuously measured the nocturnal temperature, RH,

Okamoto‑Mizuno and Mizuno ﻿Journal of Physiological Anthropology

and lighting of the gymnasium at 1-min intervals using
a thermistor, hygrometer, and illumination meter (RS12, Espec Mic Corp). We continuously measured thenoise of the gymnasium at 2-s intervals using four noise
sensors (CENTER322, Center Technology Corp.). The
floor area of the gymnasium was approximately 1050
­m2 (approximately 28 m × 37 m). The details of the
gymnasium and noise measurement areas have been
described in our previous study [5]. We continuously
measured the bed climate of the chest and foot area at
1-min intervals for five nights, during the two nights
prior to and the two nights after the school camp.
The thermistor and hygrometer sensors (RS-12, Espec
Mic Corp.) were attached to the center of the chest in
the outer layer of the clothing for the chest area andattached to the foot for the foot area. The mothers wereasked to attach the sensors to their children startingat least 30 min prior to sleep until their children woke
up. Researchers in the gymnasium attached the sensors
at the camp night one hour prior to the lights being
turned off.
One night prior to and two nights after the school
camp, we administered a subjective sleep evaluation.
The scales used for subjective sleep evaluation were as
follows: sleep onset, +2, easy; +1, slightly easy; 0, neutral; −1, slightly difficult; −2, difficult; overall sleep, +2,
good; +1, slightly good; 0, neutral; −1, slightly bad; and−2, bad. To understand the thermal, sweat, and humidity sensations, the participants were provided a questionnaire on how they felt before and after sleep. The
scales used for subjective sensations were as follows:
thermal sensation, +3, very hot; +2, hot; +1, slightly
hot; 0, neutral; −1, slightly cold; −2, cold; −3, very cold;
sweat sensation, 0, not sweating; +1, slightly sweating; +2, sweating; +3, very sweating; humid sensation,+2, humid; +1, slightly humid; 0, neutral; −1, slightly
dry; and −2, dry). As for the scales to evaluate sleep
and subjective sensations, the children were asked toanswer one discrete number on each scale. Questionnaires on bedding and clothing were asked, and a digital balance (KD-812, Tanita Corporation) was handed
out to mothers to measure the weight of their children’s clothing. On the camp day, the identical sleeping bag and mattress were handed out to the children.
The sleeping bag was made of 100% polyester with a 1.1
kg weight and 800×2100 mm in size. The dimensionsof the underside mattress were 1800 mm × 600 mm ×
8 mm. The participants answered a questionnaire after
sleep on how they used the sleeping bag before sleep,
during sleep, and upon waking: no usage, using a sleeping bag under the body, and staying inside the sleeping
bag. The participants were asked to perform their typical daily activities as usual as possible.

(2024) 43:2

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Data analysis

We re-analyzed the bedroom climate, sleep parameters,
clothing weight, and subjective assessments regarding
clothing, bedding, and bedroom conditions; subjective
sleep evaluation; and thermal and sweat sensation before
sleep from our previous study in 13 out of the 20 participants [5]. We scrutinized all data based on days before
camp (BC), the night of the camp (C), the day after camp(A1), and 2 days after camp (A2). We determined sleep
parameters using an actigram-based sleep-wake identification algorithm. We used the software Action W (AMI)
for sleep-wake scoring using Sadeh’s [12, 13] algorithm.
Furthermore, time scored as sleep was analyzed every 15
min throughout the night to determine the sleep duration. To test the statistical significance of the data, weused a one-way analysis of variance (ANOVA) (days, BC,
C, A1, and A2) for repeated measures to analyze sleep
parameters, clothing weight, and subjective sleep estimation. Additionally, a repeated measures two-way ANOVA(days and time) was used to compare temperature, RH,
light in the bedroom, sleep duration in each 15-mininterval throughout the night, bed climate, and subjective
sensations before and after sleep. Because sleep durationdiffered among the children and conditions, we examined the first 380 min, which is the shortest sleep timeobserved in A2. We used the Bonferroni method and t
test for post hoc pairwise comparisons. We set the levelof significance at P<0.05.

Results
Daytime schedule of the summer camp

The daytime schedule of the summer camp has been
described in our previous study [5]. In short, children
gathered at the school gymnasium at 16:45, slept from22:00 to 6:00, and left at 8:30 the next day.
Bedroom climate and environment of the gymnasium

Figure 1 presents the average temperature and humidityof the bedroom and the light intensity during the timespent in bed. The bedroom temperature differed significantly depending on the condition (F (3, 36) = 8.16; P <0.001) and interaction (F (57, 684) = 7.14; P < 0.001). The
bedroom temperature in BC and C was lower than thatin A1 and A2. During the initial 180 min, C was significantly higher than BC, while C was significantly lower
than A1 and A2 in the last 100 min. The bedroom temperature in C fell from 26.7°C to 25°C, while there wereno significant changes in the other conditions. The bedroom RH differed significantly depending on the condition (F (3, 36) = 29.6; P < 0.001) and interaction (F (57, 684)
= 2.90; P < 0.001). The bedroom RH in A1 and A2 wassignificantly lower than that in BC and C. During theOkamoto‑Mizuno and Mizuno ﻿Journal of Physiological Anthropology(2024) 43:2

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Fig. 1 Temperature, relative humidity, and light intensity of the bedroom and gymnasium. ▲ a significant difference (P<0.05) between C and BC; ‫־‬a significant difference between C and A1; + a significant difference between C and A2; * a significant difference between BC and A1;♦ a significant
difference between BC and A2; ‫־‬a significant difference between C and A1

initial segment of sleep, the bedroom RH in A1 was lower
than that in the other conditions, while the RH in A1
and A2 was lower than that in C and BC in the later segment of sleep. The outside mean and maximum temperatures during the day and night were as follows: BC, day,21.8/23.1°C (mean/maximum) and night, 22.8/25.3°C;
C, day, 27.8°C/30.8°C and night, 24.3 °C/27.1°C; A1, day,
28.3°C/31.5°C and night, 24.6 °C/26.1°C; and A2, day,
25.1°C/28,8°C and night, 22.8°C/24.4°C [14]. The outside temperature increased by 6°C during the day andOkamoto‑Mizuno and Mizuno ﻿Journal of Physiological Anthropology

approximately 2°C at night when comparing BC to C. We
did not observe any significant differences depending onlighting intensity. The mean noise inside the gymnasium
during the light-off period was 44.1±9.4 dB, ranging from
31.5 to 77.5 dB.
The bedding condition at home was a mattress orfuton with one covering. The same underside mattressand sleeping bag were used in C (Table 1). No significant
difference was observed in the number of layers of mattresses used by the subjects in BC, A1, and A2. Clothing
weight significantly differed depending on the condition(F (3, 36) = 4.79; P < 0.01), with significantly increased
weight in BC compared to A2.

(2024) 43:2

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Sleep parameters

Based on the wrist actigraph findings, we noted significant differences in sleep parameters between the C
and the other conditions (Table 2). No significant differences were observed in bedtimes, whereas wake-up
times (F (3, 36) = 12.11; P < 0.001) were significantly earlier in C than those in BC, and later in A1 than thosein BC and C. Additionally, the time in bed in A1 wassignificantly longer than that in C (F (3, 36) = 7.35; P <0.01). The total sleep time was significantly shorter in C
as compared to all the other conditions (F (3, 36) = 46.76;
P < 0.001). The mean activity (F (3, 36) = 95.75; P<0.001),
wake time (F (3, 36) = 69.80; P < 0.001), episodes while

Table 1 Bedroom and bedding condition during sleepa)a significant difference between BC (P<0.05)

1)2)

Number of layers of mattress and bed coverings

Table 2 Sleep parameters before and after camp

1)

Time in bed (TIB), minutes from time when subjects laid down on the bed and lights went off to wake up time; 2)TST(total sleep time),total minutes of sleep during
time in bed; 3)Wake, total minutes of wakefulness during time in bed; 4)WASO(Wake after sleep onset), minutes scored as wakefulness from sleep onset to wake up
time; 5)SEI(sleep efficiency index), SEI=Total sleep time/time in bed; 6)Mean activity, the total activity counts measured from actigraph/time in bed (min); 7)Wake
episode, total number of wake episodes; 8)Mean wake episode, wakefulness/total number of wake episode; 9)Longest wake episode, longest wake episode during
time in bed. a)a significant difference (P,0.05) between BC; b)a significant difference between C; c)a significant difference between A1

Okamoto‑Mizuno and Mizuno ﻿Journal of Physiological Anthropology

awake (F (3, 36) = 9.55; P < 0.001), mean number of
episodes while awake (F (3, 36) = 12.99; P < 0.001), thelongest episode of being awake (F (3, 36) = 38.43; P <0.001), and sleep latency significantly (F (3, 36) = 42.60;
P < 0.001) increased in C compared to the other conditions. Further, the sleep efficiency index (SEI) was significantly lower in C than in the other conditions (F (3,36) = 72.84; P < 0.001). The SEI decreased by approximately half compared to other conditions. Daytime
sleep time significantly increased in A1 as comparedto all the other conditions (F (3, 36) = 12.30; P < 0.001).
The sleep time during each 15-min period during thenight was significantly affected by the condition (F (1.4,36) = 34.72; P < 0.0001). The interaction was significant(F (46.2, 555.4) = 5.10; P < 0.001) (Fig. 2). The sleep time
was significantly lower during the first 150 min and
165 to 195 min in condition C compared to the otherconditions.
The bed climate of the chest and foot area

The changes in the bed climate temperature of the chest
and foot areas and bed climate humidity of the chest
and foot areas are depicted in Fig. 3. The bed climate temperature of the chest area differed significantly dependingon the condition (F (3, 36) =5.84; P < 0.01) and interaction (F(243, 2916) = 1.96; P < 0.001), which was significantly lower inC than that in A1 and A2. The increase in bed climate temperature in the chest area was significantly delayed in C
than in the other conditions during the initial 15 to 30 minof the sleep segment and was significantly lower than thatin A1 at 300 to 310 min. The bed climate temperature of
the foot area differed significantly depending on the condition (F (3, 36) =6.88; P < 0.001) and interaction (F (243, 2916)(2024) 43:2

Page 6 of 12=1.29; P < 0.001). The bed climate temperature of the footarea in BC and C was significantly lower than that in A1.
The temperature increase in the bed foot area was also significantly delayed in C compared to other conditions during the initial 5 to 15 min of sleep. Further, the bed climate
temperature of the foot area in BC was significantly lower
than that in A1 and A2 before the lights were off. A significantly lower temperature in C than in A2 was observed
intermittently from 60 to 210 min, while a significant difference between C and A1 was observed several minutes
after the 15 min. There was a significantly lower temperature in BC than in A2 and A1 observed intermittently from170 to 240 min. The bed climate RH of the chest area differed significantly depending on the interaction (F (243, 2916)=2.15; P < 0.001). The bed climate RH of the chest area
was significantly higher in C as compared to that in A1 andBC during the initial 10 to 15 min of sleep. The bed climate
RH of the foot area differed significantly depending on thecondition (F (3, 36) =30.0; P < 0.001) and interaction (F (243,2916) =1.46; P < 0.001), which was significantly higher in BC
and C as compared to A1 and A2. The bed climate humidity of the foot area was significantly higher in BC and C
than in A1 intermittently throughout the night, while it
was higher in BC than in A2 during the later sleep segment. No significant differences were observed in the bed
climate absolute humidity of the chest and foot areas.
Subjective assessments

Regarding the usage of the sleeping bag in C, 15% ofparticipants spent time inside the sleeping bag before
going to sleep, and this figure increased to 50% in thelater segment of sleep before the wake-up time (Fig. 4).
We noted significant differences among the conditions

Fig. 2 The sleep time during each 15-min period during the night. *P<0.05 a significant difference between C and other conditions

Okamoto‑Mizuno and Mizuno ﻿Journal of Physiological Anthropology(2024) 43:2

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Fig. 3 Bed climate of the chest and foot area. ▲ a significant difference (P<0.05) between C and BC; ‫ ־‬a significant difference between C and A1; +a significant difference between C and A2; * a significant difference between BC and A1; ♦ a significant difference between BC and A2

Okamoto‑Mizuno and Mizuno ﻿Journal of Physiological Anthropology(2024) 43:2

Page 8 of 1260
50

No usage
Sleeping bag under the bodyGet inside the sleeping bag

40

%

30
20
10
0
Before Sleep

During Sleep

Upon Waking

Fig. 4 The usage of a sleeping bag during sleepin the subjective overall assessments of sleep (F (3, 36)=13.30; P < 0.001) and sleep onset (F (3, 36) =15.53; P <0.001). The subjective overall assessment of sleep was significantly worse, and sleep onset latency was longer in C
than in the other conditions (Fig. 5). Regarding subjective sensations before and after sleep, a significant effectof condition, time, and interaction was observed forsweat (condition, F (3, 36) = 4.01; P < 0.001; time, F (1,12)=6.36; P < 0.001; interaction, F (2.2, 27.3) =3.03; P < 0.001)
and humid sensations (condition, F (3, 36) =3.51; P < 0.001;
time, F (1,12) =8.15; P < 0.001; interaction, F (3, 36) =3.03; P< 0.001). Sweat sensations increased in C as compared tothat in BC, before sleep, and increased in C as comparedto that in A2, after sleep. The humidity before sleep was
significantly higher in C than that in BC and A2. Additionally, a significant effect of condition (F (3, 36) =14.9; P< 0.001) and interaction (F (1.9, 22.8) =3.71; P < 0.001) was
observed for thermal sensations. The thermal sensation
before sleep was significantly higher in C than that in
BC. The most frequent reason for sleep disturbances during camping was heat (n=10). Besides heat, noise [4], ahard floor [3], a small space [2], and collisions [2] wereobserved with none of the children claiming anxiety.

Discussion
In agreement with the results of our previous study[5], sleep was disturbed in C compared to that inother conditions, and heat was identified as the mostfrequent sleep disturbance, as reported by 10 of the

13 participants. The sleep disturbance in C was concentrated during the initial 195 min. One reason for
the increased heat stress may be related to the ambient climate in the initial segment of sleep. The ambient
temperature was significantly lower in BC than in A1
and A2, while the significant difference between C andBC was limited during the initial 180 min. The ambient temperature in the initial 180 min in C was accompanied by significantly higher ambient RH than that in
A1 and A2, which may have increased heat stress. The
subjective sensation before sleep may further support
this finding. The subjective humid sensation before
sleep was significantly wet in C compared to BC andA2. Furthermore, subjective sweat and hot sensations
before sleep were significantly higher in C than in BC.
Considering that 85% of the participants did not sleep
inside the sleeping bag at sleep onset, it is possible thatthe temperature and RH during the initial sleep segment may have been perceived by the children as hot.
The hot sensation may also be related to cramping up
and collisions, and no air flow due to thick curtains on
the windows in the gymnasium, leading to higher ambient temperature and humidity in the areas near eachparticipant, as suggested in our previous study [5].
Another factor that should be considered is that theoutside temperature during the day and night increased
abruptly from BC to C. Although the ambient temperature in C was higher than 26°C in the initial 180 min,it gradually decreased to 25°C which was significantly

Okamoto‑Mizuno and Mizuno ﻿Journal of Physiological Anthropology(2024) 43:2

Page 9 of 12

Sweat Sensation
BC

*

C
A1

Before S
After S
Sweating

*

A2
0

1

2

3

Thermal Sensation
BC

Before S
After S

C

**

A1
A2

Very Cool-3

Very Hot

-2

-1

0

1

2

3

Humid Sensation
BC

Before S
After S

C

*
*

A1
A2

Dry

-2

Humid

-1

0

Sleep Onset

1

2
Easy

Difficult

BC

**

C

**

A1

**

A2
-2

-1

0

Overall Sleep

1
Good

Bad

BC

2

**

C

**

A1
A2
-2

-1

0

Fig. 5 Thermal sensation during sleep and subjective sleep evaluation. *P<0.05, **P<0.01

1

2

**

Okamoto‑Mizuno and Mizuno ﻿Journal of Physiological Anthropology

lower than that in A1 and A2 in the last sleep segment. We did not observe this ambient temperature
under other conditions, which suggests that the ambient temperature inside the gymnasium might be more
affected by the outside temperature. One reason forthis change could be the construction of the gymnasium in which the floor bottom is usually outside in
the open air, whereas heat insulators generally exist in
homes. One possibility for the absence of a significant
difference in sleep time after 195 min might be related
to the decreased ambient temperature, since it started
to decrease below 26 °C from 180 min. However,decreased sleep time during the initial 195 min may
include not only ambient climate but other factors suchas the first night effect, the effect of noise, and hard
floors, as mentioned in our previous study [5]. Further,no significant difference after 195 min may also include
factors besides ambient climates such as increased
sleep pressure and decreased noise level [5]. These factors should be considered and need caution when considering the relationship between ambient temperatureand sleep in C. However, considering that controllingthe ambient temperature in the gymnasium by measuring ambient temperature near each participant is difficult, our study at least indicates the possibility thatambient temperature should be controlled lower than
26 °C during the initial segment of sleep when morethan 170 people are staying in the gymnasium.
The disturbed sleep in C was related to a significantly
delayed increase in the bed climate temperature in
the chest area in C compared to other conditions during the initial 15 to 30 min. One possible explanation
for this outcome may be related to the fact that children did not sleep inside the sleeping bag, resulting
in low bed climate temperature in the chest area. Past
studies on awake subjects imply that the sweating rate
is lower in children than in adults [15, 16]. However,children use distinct mechanisms for thermoregulation in hot environments because of greater cutaneous
vasodilation of the head and trunk and a greater surfacearea-to-mass ratio [15, 16]. This suggests that children
rely more on dry heat loss than evaporative cooling.
Although behavioral thermoregulation of not entering
the sleeping bag and sleeping directly on the underside
mattress may have increased dry heat loss, it might nothave been sufficient to overcome heat stress in C. A
previous study on adults found that the covered area ofthe body, by bed covering during sleep, differs depending on ambient temperature, which is more sensitivein the upper extremities than in the lower extremities
or trunk, and the lower extremities are more sensitive than the trunk [17]. This suggests that behavioral
thermoregulation is active during sleep. A previous(2024) 43:2

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study indicated that behavioral thermoregulation during sleep might be more important in children than in
adults [18]. In sleeping bags, behavioral thermoregulation is limited to covering the trunk and lower extremities, or not covering both. Furthermore, staying inside asleeping bag may limit body movement, sleep positionchanges, and ventilation inside the bed climate. This
constraint may further increase the thermal stress andexplain the behavior in C. Considering that the ambient temperatures in the gymnasium can be influenced
by outside temperatures and that sleeping bags mayhinder behavioral thermoregulation and dry heat loss,
different kinds of bedding should be provided to thechildren in the gymnasium.
Another possibility may be related to disturbed sleep
resulting in thermoregulation changes during sleep
onset. During the normal sleep onset period in adults,
chest and foot skin temperatures and bed climate temperatures increase and are maintained throughout thenight [18]. Increased skin temperature (Tsk) and bed climate temperature during the sleep onset period have also
been observed in school-aged children [18, 19] and preschool children with normal sleep [10]. Further, previous
research and the present study imply that the bed climate
temperature increases during the sleep onset period andis maintained throughout the night in school-aged children [10]. However, the Tsk increase during sleep onset
may differ between adults and preschool children. Preschool children may rely on chest Tsk for heat dissipation at sleep onset, followed by increases in foot Tsk [10].
In school-aged children, chest Tsk also increases to its
maximum level earlier than foot Tsk at sleep onset. Further, foot Tsk at sleep onset takes 90 min to reach the plateau level [19]. This might explain why the significantly
delayed temperature increase in bed climate temperature
in C compared to other conditions was shorter in the footarea than in chest area. Children’s cutaneous vasodilationof the head and trunk increases in hot environments during a waking state [15, 16]. Thus, increasing proximal Tsk
may also be important for school-aged children during
sleep onset to decrease cardiovascular strain during sleep[10]. Since the sleep onset period was excessively delayedin C compared to that in the other conditions, the chest
Tsk may not have increased at sleep onset, resulting in alower bed climate temperature of the chest at the initial
15 to 30 min. Considering that sweat sensation before
sleep was higher in C than in BC and bed climate humidity of the chest area at initial 5 to 10 min was significantly higher in C than in BC and A1, sweating may havedecreased bed climate increase in C.
Although sleep time was lower in C than in other conditions during the initial 195 min, a significant difference
in bed climate temperature of the chest area between C

Okamoto‑Mizuno and Mizuno ﻿Journal of Physiological Anthropology

and the other conditions was limited to the initial 30 min.
This might indicate that regardless of sleep or wakefulness, the bed climate temperature of the chest area does
not differ after the sleep onset period. This result might
be attributed to the Tsk change during sleep in children.
The chest Tsk increases to its maximum at sleep onset
and gradually decreases until the timing when the footTsk reaches a plateau [19]. The bed climate temperatureof the chest area in A1 and A2 also decreased after maximum, and BC maintained a plateau, while it continued toincrease in C, leading to no significant difference among
the conditions. The Tsk increases due to increased skinblood flow, which has two pathways, one is the noradrenergic vasoconstrictor system, and the other is theactive vasodilator system. The Tsk increase at sleep
onset is attributed primarily to reduced activation ofnoradrenergic vasoconstrictor tone [20], while increased
Tsk under heat stress during wake is due largely to anincreased active vasodilator system [21]. Although wecannot distinguish these two pathways, the bed climate
temperature increase in the chest area in C might largely
be related to the Tsk increase due to the active vasodilator system, at least during the initial 195 min. Behavioral thermoregulation in C may have also accounted
for no significant difference in bed climate temperatureof the chest area among the conditions. Although 85%
of the children were not inside their sleeping bags, 50%
were sleeping inside their sleeping bags upon waking.
Since body temperature generally decreases during sleep,
ambient temperature decreases in C until 25°C may havebeen perceived as cool by some children in the morning,leading to increased covering behavior and bed climate
temperatures in the later sleep segment. Since children
rely on dry heat loss, covering and uncovering behaviors
in relation to sleeping bags may have supported homeothermy during later segment of sleep in C.
Time is needed to improve Japanese shelters, as it
requires revising both the political system itself and thelaw. However, the outside temperature is increasing year
by year [22]; for example, during summer in 2023, themaximum lowest outside temperature was 27.9°C, themaximum temperature was 39.8°C, and the mean RH
was 75%, in Osaka, which is one of the hottest prefecturesin Japan [14]. Our results indicate the urgent need to atleast set up portable air conditioning in gymnasiums andpreparing a system to supply portable beds, mattresses,
and bed coverings throughout Japan.
This study has several limitations that should be considered. First, none of our participants claimed anxiety
as a sleep disturbance factor since this was an enjoyable
event. This signals that mental stress is completely different from real situations involving natural disasters.

(2024) 43:2

Page 11 of 12

Second, the number of evacuees in a shelter is generally much greater than in this study, and the effect may
increase further. Third, questionnaires about how theyused the sleeping bag before and during sleep wererecalled after sleep. This might lack accuracy, since theanswers are limited to the memories when participants
were awake. Fourth, the age and number of participants
were limited in this study. Larger sample sizes anddifferent age groups should be considered for future
research. Fifth, the results of this study are unsuitable for generalizing because the ambient temperatureand humidity inside the gymnasium can be influenced
by outside temperatures and the number of people
present.

Conclusions
In summary, these outcomes reveal that a noticeable
delay in bed climate temperature increase in the chest
area during the sleep onset period may be associated withsleep disturbances in children sleeping in a simulated
shelter. Setting up a portable air conditioning in the gymnasium and preparing a system to supply portable beds,
mattresses, and bed coverings are necessary measuresthat should be taken to avoid sleep disturbances in children as well as all the evacuees during future disasters.
Abbreviations
RH	Relative humidity
BC	Night before school campC	School camp
Tsk	Skin temperature
A1	One day after school camp
A2	Two days after school camp
Acknowledgements
This study was supported by the Program of Funding Basic Research Centers
in Private University (MEXT) to the Kansei Fukushi Research Institute, Tohoku
Fukushi University.
Authors’ contributions
All authors contributed to the study conception and design, and data collec‑
tion. Data analysis was performed by KO. The first draft of the manuscript was
written by KO, and KM commented on previous versions of the manuscript. All
authors read and approved the final manuscript.
Funding
This study was supported by the Program of Funding Basic Research Centers
in Private University (MEXT) to the Kansei Fukushi Research Institute, Tohoku
Fukushi University.
Availability of data and materials
The datasets during and/or analyzed during the current study are available
from the corresponding author on reasonable request.

Declarations
Ethics approval and consent to participate
This study was approved by the Ethics Committee of Tohoku Fukushi
University.

Okamoto‑Mizuno and Mizuno ﻿Journal of Physiological Anthropology

Consent for publication
Consent for publication was informed to the participants and their parents,
prior to giving their written consent. All the participants and their parents
gave their written consent.
Competing interests
The authors declare that they have no competing interests.
Received: 5 June 2023 Accepted: 6 December 2023

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