Maeda et al.
Journal of Physiological Anthropology(2023) 42:20
https://doi.org/10.1186/s40101-023-00337-0

Journal of
Physiological Anthropology

Open Access

ORIGINAL ARTICLE

Effects of bathing‑induced changes in bodytemperature on sleep
Takafumi Maeda1,2* , Hiroko Koga3, Takashi Nonaka3 and Shigekazu Higuchi1,2

Abstract
Background Passive body heating before sleep is well known to lead to improved sleep. However, the effectsof the degree of change in body temperature by bathing on sleep quality are unclear. The present study aimed
to clarify the effects on sleep of bathing-induced changes in body temperature.
Methods Twenty-three healthy males and females in their 20 s to 50 s bathed in their homes 1.5–2 h before bedtime
under three bathing conditions: showering only; short bathing in a bathtub; and long bathing in a bathtub. Sublingual and skin temperatures and thermal sensation before and after bathing, sleep indices such as sleep onset latency,
time in bed, sleep efficiency, and wake after sleep onset, all of which were evaluated using an actimeter, and subjective evaluations of sleep were compared among conditions.
Results Sublingual temperature just after bathing was significantly higher with long bathing than with other conditions, and the fall in sublingual temperature from after bathing to before sleep was significantly larger with long bathing than with short bathing. Sleep onset latency by actimeter was significantly reduced with long bathing compared
to showering. In addition, subjective evaluations of falling asleep and sleep quality were better with long bathing
than with showering or short bathing.
Conclusions In conclusion, bathing conditions that produce a 0.9 °C increase in sublingual temperature appear
effective for falling asleep and sleep quality, because core temperature shows a greater drop to before sleep
than those producing an increase of about 0.3 °C increase in sublingual temperature.
Keywords Bathing, Body temperature, Sleep onset latency, Sleep quality, Distal-to-proximal skin temperature
gradient
Background
Many people, especially working age individuals in their
20 s to 50 s, are dissatisfied with sleep [1]. Sleep is disturbed by various factors, one of which sleep onset*Correspondence:
Takafumi Maeda
maeda@design.kyushu-u.ac.jp
1
Department of Human Life Design and Science, Faculty of Design,
Kyushu University, 4‑9‑1, Shiobaru, Minami‑Ku, Fukuoka 815‑8540, Japan2
Physiological Anthropology Research Center, Faculty of Design, Kyushu
University, 4‑9‑1, Shiobaru, Minami‑Ku, Fukuoka 815‑8540, Japan
3
Research & Development Division, Noritz Corporation, 5,
Minami‑Futami, Futamicho, Akashi, Hyogo 674‑0093, Japanlatency (SL) is reportedly prolonged in persons with low
skin temperature in the peripheral extremities [2]. Bathing is known to have positive effects on sleep, and various studies have attempted to clarify the relationshipbetween bathing and sleep-related variables [3–12].
The effects of bathing timing on sleep have been examined in a number of investigations. Bunnell et al. [4]investigated the effects on sleep of the timing of bathing,comparing bathing in the morning (within 1 h of waking), afternoon (10 h before bedtime), evening (6 h before
bedtime), and late night (immediately before bedtime).
SL was shown to be shortened by bathing in the evening[4]. Inagaki et al. [8] reported that bathing 2 h before bedtime was more effective in promoting sleep quality than

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Maeda et al. Journal of Physiological Anthropology(2023) 42:20

bathing 0.5 or 1 h before bedtime. Some studies have also
reported that bathing less than 2 h before bedtime, suchas 1.75 to 2 h [6] or 1.5 h [5] before bedtime, has positiveeffects on sleep.
A systematic review by Haghayegh et al. [13] foundthat passive body warming by a hot shower, foot bathfor 10 min or more, or whole-body bathing within about
1–2 h before bedtime resulted in shorter SL, increased
sleep efficiency, and improved subjective sleep quality.
However, that review included 17 studies on footbathand sleep, but only 6 studies on whole-body bathing andsleep. Four of those 6 studies with whole-body bathingtargeted the elderly, and the remaining 2 studies targeted
only young people in their 20 s [3, 9]. Expanding the target group from young people to middle-aged people istherefore necessary, because improving sleep quality inworking age individuals in their 20 s to 50 s is inducing
the increased productivity [14–16]. In addition, althoughmany studies have compared bathing and non-bathing
conditions [6, 10], comparisons between different bathing (e.g., showering) conditions appear warranted, given
the bathing habits of the young to middle-aged Japanese
population, in which the ratio of showering is high [17].
Many of the studies on bathing and sleep that havereported positive effects on sleep from bathing before
going to bed have been laboratory investigations [3, 6, 9,10]. In laboratory experiments, overnight polysomnography can be used to evaluate sleep and thus can be used
as an index, but the novel sleep environment itself may
affect sleep. On the other hand, studies conducted in living spaces of participants have often evaluated sleep subjectively [11, 18, 19], and objective evaluations of sleepin actual living spaces are therefore needed. Kanda et al.
[18] evaluated sleep objectively using a body motion sensor placed under the bed for young and elderly people
in actual living spaces, revealing that bathing improved
sleep quality compared with non-bathing. Tai et al. [11]conducted a cross-sectional evaluation of the effectsof bathing on sleep using an actimeter in an actual living space, and clarified that bathing 60–180 min before
bedtime shortens sleep onset latency. However, thoseprevious studies did not control for bathing conditions in
actual life and were unable to clarify the effects on sleepof bathing-induced changes in body temperature.
According to Haghayegh et al. [13], SL is related to themagnitude of the decrease in core temperature after body
heating by bathing etc., rather than any specific value ofcore temperature itself due to body heating before going
to bed. In other words, a greater decrease in core temperature after bathing appears important for improving
SL. However, a systematic review by Haghayegh et al.
[13] included no descriptions of the effects on sleep ofPage 2 of 10

differences in bathing time or degree of increase in coretemperature due to bathing.
Regarding rises in body temperature due to bathing, a study by Dorsey et al. [6] on elderly women withinsomnia found that bathing to the mid-thorax level in
40–40.5 °C water for 30 min between 1.75 and 2 h before
bedtime increased rectal temperature by 0.71 ± 0.31 °C.
They reported that under those bathing conditions, halfof the participants showed an increase in sleep maintenance (SM), a reduction in wake after sleep onset(WASO), and an increase in slow-wave sleep compared
to a night without bathing. In a study by Mishima et al.
[10] on elderly people with vascular dementia, core temperature decreased by an average of about 0.8 °C fromimmediately after bathing to the mid-thorax level in 40 °C
water for 30 min at 2 h before bedtime to bedtime and SL
was significantly reduced under bathing conditions compared to the baseline night without bathing. These studies indicate a relationship between the degree of decreasein core temperature after bathing and sleep. However, theeffects on sleep of the difference in core temperature riseassociated with bathing have not been clarified.
The purpose of the present study was to clarify theeffects of the degree of increase in core temperature withbathing on SL (falling asleep) and sleep quality using field
experiments in actual life spaces, targeting a wide rangeof age groups from young to middle-aged.

Methods
Participants

Twenty-three healthy adults with regular sleeping habits participated in this study (12 males, 11 females; mean[± standard deviation] age, 39.3 ± 10.4 years; height,
165.6 ± 12.2 cm; mass, 59.0 ± 11.7 kg; body mass index,
21.4 ± 2.3 kg/m2). Participants ranged in age from 23 to
54 years, with 6 participants in their 20 s, 5 in their 30 s,
7 in their 40 s, and 5 in their 50 s. Shift or night workers,
and individuals taking medications for sleep disturbancewere excluded from the present study. Written informed
consent was obtained from all participants prior to enrolment in the study. All research protocols were approvedby the Ethics Committee of Noritz Corp (approval no.
0014). Participants abstained from heavy work, strenuous
exercise, and alcohol consumption from the day before
the experiment, abstained from smoking and napping on
the day of the experiment, and abstained from caffeine
intake from 18:00 prior to the scheduled tests.
According to a preliminary questionnaire, 18 participants reported trouble or dissatisfaction with sleep.
Mean score for the Japanese version of the Pittsburgh
Sleep Quality Index [20] was 6.3 ± 2.0, and 18 participants
showed scores ≥ 5. In addition, 14 participants reported
frequently feeling that their hands and/or feet were cold.

Maeda et al. Journal of Physiological Anthropology(2023) 42:20

For female participants, the stage of the menstrual cycle(follicular or luteal phase) was not controlled for, but wasgauged based on a questionnaire. We confirmed that the
number of participants in each stage of the menstrual
cycle did not differ significantly between conditions.
Experimental environment and condition

The experiment was conducted in the home of each participant, to eliminate the effects of unfamiliar environments in the artificial climatic chamber on sleep. The
study was performed from December 2020 to March
2021.
A systematic review [13] confirmed that the increase inperipheral skin temperature of the hands and feet due tobathing and the decrease in core temperature after bathing were associated with shorter SL. This study therefore
set the following three conditions: a showering condition(showering) in which neither skin temperature nor coretemperature rose easily; a short bathing condition in abathtub (short bathing) in which skin temperature roseand core temperature rose slightly; and a long bathing
condition (long bathing) in which both skin temperatureand core temperature rose.
Under the showering condition, only the face, body,and hair were washed in the shower. Under the bathing conditions, the face, body, and hair were washed byshowering, followed by full-body bathing with waterlevel up to the mid-thorax level in a long sitting position.
Water temperature in the shower and bathtub was set at
about 40 °C under all conditions [5, 6, 8, 10].
The rise in core temperature due to bathing differs
depending on the bathing environment and individual
characteristics [5, 21, 22], even when bathing time is thesame. In the present study, bathing time was determinedin advance for each participant with a target rise in rectal
temperature of about 0.5 °C in the long bathing condition
and about 0.1 °C in the short bathing condition, whichbased on a predicted rectal temperature change using thethermal model for bathing [23]. The target rise of about
0.5 °C was set, in consideration of safety, referring to aprevious study in which core temperature rose by about
0.7 °C [6, 10, 23]. Bathing duration was 5.2 ± 1.4 min for
short bathing and 16.1 ± 3.0 min for long bathing.
Bathing was started 1.5–2 h before going to bed, basedon previous studies [5, 6, 8]. Air temperature in thebathroom was not controlled, but did not differ significantly among conditions, at 17.3 ± 4.8 °C for showering,
18.2 ± 4.2 °C for short bathing, and 17.5 ± 4.7 °C for longbathing.
Each participant wore their own same sleep clothes andsocks from after bathing until going to bed. Participants
were instructed that should be no difference in the use ofheating appliances such as hot carpets and floor heating

Page 3 of 10

after bathing under the three conditions, and that theyshould go to bed and wake up at their usual times. Participants were also restricted from using electric blankets
while sleeping. Air temperature in the bedroom was notcontrolled, but did not differ significantly between conditions, at 17.4 ± 3.2 °C for showering, 17.8 ± 3.0 °C for shortbathing, and 17.8 ± 3.5 °C for long bathing.
Experimental procedure

The experiments were conducted in the home of the participant on the night of a working day, excluding holiday
or days before holidays. The three bathing conditions
were applied in random order. The experiment for the
first bathing condition was conducted for 4 days within
1 week, with the first day set as the acclimation day andthe next 3 days as observation days. The experiment forthe second and third conditions was conducted for 3 days
within 1 week for each condition as observation days.
Among the observation days, 1 day that satisfied the conditions for bathing timing and temperature was used foranalysis.
Measurements
Sleep parameters

Activity during sleep was recorded using an actimeter(FS-770; Acos, Japan) attached to the waist to objectively
evaluate sleep states from posture changes during sleep.
Sleep indices such as SL, time in bed (TIB), total sleep
time (TST), sleep efficiency (SE), and WASO were estimated from activity data using a sleep–wake rhythm app(SleepSign Act version 2.0; Kissei Comtec, Japan) [24]. SE
was calculated by formula SE = TST/TIB. The concordance rate between this system and polysomnography forsleep/wake judgment has been reported as 88.4% [24].
Subjective feelings of falling asleep and quality of sleep

In terms of subjective feelings regarding sleep, falling
asleep and quality of sleep were measured by the semantic differential method using a 9-point scale (from 9,“very good” to 1, “very bad”) after waking.
Oguri‑Shirakawa‑Azumi sleep inventory

Subjective sleep quality was also assessed using theOguri-Shirakawa-Azumi Sleep Inventory Middle aged
and Aged version (OSA-MA) [25] the morning after eachexperimental night. The OSA-MA is a standardized sleep
inventory comprising 16 items, answered using a 4-point
scale. The 16 items can be categorized into the following
five factors: Factor I, “sleepiness on rising"; Factor II, “initiation and maintenance of sleep"; Factor III, “frequent
dreaming"; Factor IV, “refreshing"; and Factor V, “sleep
length”. For all factors, a higher score indicates better

Maeda et al. Journal of Physiological Anthropology(2023) 42:20

sleep. The OSA-MA is commonly used in sleep studies ofJapanese participants [26–31].
Sublingual and skin temperatures

Sublingual temperature was measured before and after
bathing, and before sleep using a basal body thermometer (MC-652LC; Omron, Japan) to confirm changes incore temperature.
Skin temperatures at the trunk (forehead, chest) andperipheral parts (back of the hand and instep) of the body
were measured before and after bathing and before sleep
using an infrared thermometer (S-708; Seastar, Japan) tocheck changes in skin temperature. Distal-to-proximal
skin temperature gradient (DPG) is an indirect index ofvasodilation in the distal skin [4] that is reportedly associated with SL [32], and was calculated by subtracting
chest skin temperature from the instep skin temperature.
Thermal sensations before sleep

Thermal sensations considering overall body and local
sensations in the hands and feet were measured using a
7-point scale (7, very hot; 6, hot; 5, warm; 4, neutral; 3,cool; 2, cold; and 1, very cold) before going to sleep.
Data screening and analysis

The following days were excluded from analysis: thefirst day of the experiment (the acclimation day); any
day in which bathing time started within 1.5 h or 2 h or
more before bedtime; and days on which the participant
experienced extreme stress or poor physical condition(headache, etc.). In addition, the following days were prioritized, and one day was analyzed for each condition: forthe same participant (between conditions), days after thesecond day of each condition; days with almost the sametimes of bedtime and wake-up time, and days with thesame workplace (office/home).
Statistical analyses were performed using SPSS version
22.0 (IBM SPSS Statistics, USA). The Shapiro–Wilk test
was performed to confirm the normality of data. In termsof physiological measurements with confirmed normality
such as body temperature, blood pressure, and heart rate,
two-way analysis of variance (ANOVA) with repeatedmeasures was used to examine the main effects of condition and time, as well as interactions between both, followed by post-hoc testing using Bonferroni correction.
In terms of each sleep index obtained from the actimeter, total score and each factor score on the OSA-MA andsleep-related subjective measures for which normality
was not confirmed, the Friedman test was used to examine the main effect of conditions, and the Wilcoxon test

Page 4 of 10with Bonferroni correction was used to detect differences
between each condition as post-hoc testing.
Value of p < 0.05 were considered statistically significant. All data are expressed as mean ± standard deviation.

Results
Sleep parameters

Bedtimes did not differ significantly among conditions, at
0:27 ± 1 h 10 min for showering, 0:26 ± 1 h 6 min for shortbathing, and 0:29 ± 1 h 6 min for long bathing. Timesout of bed likewise did not differ among conditions, at
6:57 ± 33 min for showering, 6:58 ± 42 min for short bathing, and 6:50 ± 34 min for long bathing.
Figure 1 indicates the results of sleep parametersassessed using the actimeter. SL was 20.3 ± 14.3 minwith showering, 16.1 ± 13.9 min with short bathing, and12.3 ± 10.1 min with long bathing (Fig. 1a). SL was significantly shorter with long bathing than with showering (p < 0.05). TIB was 389.5 ± 67.6 min with showering,392.5 ± 61.9 min with short bathing, and 381.6 ± 62.4 minwith long bathing (Fig. 1b). No significant differences in
TIB were seen among conditions. SE was 78.69 ± 11.06%
with showering, 80.30 ± 9.75% with short bathing, and82.05 ± 9.60% with long bathing (Fig. 1c). No significant
differences in SE were seen among conditions. WASO
was 52.3 ± 36.4 min with showering, 50.5 ± 34.8 min withshort bathing, and 44.1 ± 32.4 min with long bathing(Fig. 1d). No significant differences in WASO were seenamong conditions.
Subjective feeling of falling asleep and quality of sleep

Figure 2 indicates the results from subjective evaluationsof falling asleep and quality of sleep. Falling asleep andquality of sleep were significantly better with long bathing than with other conditions (falling asleep: vs showering, p = 0.003; vs short bathing, p = 0.024; sleep quality: vsshowering, p = 0.001; vs short bathing, p = 0.002).
OSA‑MA sleep score

Figure 3 indicates each factor for OSA-MA (Fig. 3a) andeach item in Factor II, which is related to the initiationand maintenance of sleep (Fig. 3b). In terms of each factor for OSA-MA, no differences were seen in Factors I,
III, IV, or V, but significant differences in scores wereseen for Factor II (p < 0.001), with significantly higherscores for long bathing than for other conditions (vsshowering, p < 0.001; vs short bathing, p = 0.045) (Fig. 3a).
Concerning the contents of Factor II, SM score differedsignificantly between conditions (p = 0.001) by ANOVA,
and post-hoc testing showed higher scores with longbathing than with showering (p = 0.002) (Fig. 3b). Sleep

Maeda et al. Journal of Physiological Anthropology(2023) 42:20

Page 5 of 10

Fig. 1 Sleep parameters assessed using the actimeter. a Sleep onset latency; b time in bed; c sleep efficiency; and d wake after sleep onset.
*Significant difference, p < 0.05

Sublingual and skin temperatures and DPG

Fig. 2 Subjective feeling of falling asleep and quality of sleep.
Squares, triangles, and circles indicate showering and short andlong bathing conditions, respectively. ** Significant difference
from showering condition, p < 0.01. # Significant difference
from short bathing condition, p < 0.05. ## Significant difference
from short bathing condition, p < 0.01

onset score for Factor II differed significantly between
conditions (p < 0.001) and post-hoc testing showed significantly higher scores with long bathing than with otherconditions (vs showering, p = 0.001; vs short bathing,p = 0.024) (Fig. 3b).

Table 1 shows sublingual temperature, skin temperatures(forehead, chest, back of the hand, and instep) and DPG
before and after bathing and before sleep, changes frombefore to after bathing, and changes from after bathing tobefore sleep.
Regarding sublingual temperature, significant main
effects of bathing condition (p = 0.037) and time(p < 0.001) and a significant interaction (p < 0.001) wereseen. In all conditions, sublingual temperature increased
significantly from pre-bathing to post-bathing (p < 0.001),
then decreased significantly until before sleep (p < 0.001).
Sublingual temperature at pre-bathing was significantly lower with long bathing than with short bathing(p = 0.027). After bathing, sublingual temperature wassignificantly higher with long bathing than with showering or short bathing (p < 0.001 each). Before sleep, no
significant difference was observed between bathing
conditions.
The rise in sublingual temperature associated withbathing (“B-A” in Table 1) differed significantly between
conditions (F = 40.286, p < 0.001), with long bathing
showing a significantly greater rise than the other two
conditions (p < 0.001 each). The fall in sublingual temperature from bathing to sleep (“C-B” in Table 1) differedsignificantly between conditions (F = 8.130, p = 0.002),
showing a significantly greater fall with long bathing than

Maeda et al. Journal of Physiological Anthropology(2023) 42:20

Page 6 of 10

Fig. 3 OSA-MA score. a Five factors of OSA-MA. b Contents in Factor II (initiation and maintenance of sleep). OSA-MA, Oguri–Shirakawa–Azumi
Sleep Inventory, Middle-aged and Aged version. Grey, orange, and red marks indicate showering and short and long bathing conditions,
respectively. *Significant difference, p < 0.05. **Significant difference, p < 0.01

with short bathing (p = 0.005) and a tendency to decrease
with long bathing compared with showering (p = 0.066).
Regarding skin temperatures, an interaction between
bathing condition and time was evident for skin temperatures at the chest (F = 3.432, p = 0.023), back of the hand
(F = 3.164, p = 0.046), and instep (F = 5.510, p = 0.003). As
a result of post-hoc testing, compared with showering,instep skin temperatures after bathing were significantly
higher with both short and long bathing (p < 0.001 each),and significantly higher before sleep with long bathing(p = 0.010). The increase in skin temperature associatedwith bathing differed significantly between bathing conditions only at the instep, and the rise in skin temperature
at the instep due to bathing (B-A) was significantly higher
with both short and long bathing than with showering(short bathing, p = 0.007; long bathing, p < 0.001). The fallin forehead skin temperature from after bathing to before
sleeping (C-B) was significantly higher with long bathing
than with short bathing (p = 0.041), but no differences inother skin temperatures were noted between conditions.

Regarding DPG, representing the difference in skintemperatures between the instep and chest, main effectsof bathing condition (F = 7.125, p = 0.002) and time
(F = 23.595, p < 0.001) and an interaction between bathing condition and time (F = 3.380, p = 0.013) were seen.
Although no significant differences in DPG were seen
before bathing, DPG just after bathing was significantly
higher with short and long bathing compared to showering (p < 0.001 each), and also significantly higher withlong bathing than with short bathing (p = 0.044). Furthermore, DPG before sleep was significantly higher
with long bathing than with showering (p = 0.019). The
rise in DPG associated with bathing (B-A) differed significantly between conditions (F = 7.644, p = 0.001), andthe rise was significantly higher with long bathing thanwith showering (p = 0.001). The fall in DPG from bathing
to sleep (C-B) did not differ significantly between conditions (F = 0.603, p = 0.552).

Maeda et al. Journal of Physiological Anthropology(2023) 42:20

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Table 1 Changes in sublingual and skin temperatures and DPG(°C)
Part

Conditions

Before bathing (A)After bathing (B)Before sleep (C)

B-A

C-B

SublingualShowering
Short bathing
Long bathing

36.48 ± 0.36
36.55 ± 0.28
36.34 ± 0.38#

36.83 ± 0.34
36.82 ± 0.30
37.25 ± 0.45**,##

36.17 ± 0.34
36.28 ± 0.35
36.31 ± 0.22

0.35 ± 0.32
0.27 ± 0.23
0.91 ± 0.40**,##− 0.66 ± 0.23
− 0.55 ± 0.26
− 0.94 ± 0.49##

Forehead

Showering
Short bathing
Long bathing

32.83 ± 0.91
32.94 ± 0.97
32.49 ± 1.09

33.03 ± 1.12
33.35 ± 1.26
32.76 ± 1.22

32.89 ± 1.34
32.89 ± 1.34
32.97 ± 1.46

0.20 ± 1.05
0.41 ± 1.24
0.27 ± 1.22

− 0.14 ± 0.93
− 0.47 ± 1.21
0.21 ± 1.05#

Chest

Showering
Short bathing
Long bathing

33.87 ± 0.76
33.32 ± 1.19*
33.17 ± 1.33*

33.43 ± 1.42
33.41 ± 0.95
32.91 ± 1.42

33.70 ± 1.32
33.90 ± 1.21
33.93 ± 1.03

− 0.44 ± 1.44
0.09 ± 1.21
− 0.26 ± 1.34

0.27 ± 1.08
0.49 ± 1.24
1.03 ± 1.24

Backof the handShowering
Short bathing
Long bathing

30.07 ± 2.59
30.07 ± 2.12
29.09 ± 2.80

32.10 ± 1.30
32.37 ± 1.34
32.31 ± 1.39

29.99 ± 3.49
31.37 ± 2.38
31.65 ± 2.11

2.03 ± 2.61
2.30 ± 2.27
3.23 ± 3.22

− 2.10 ± 3.36
− 1.00 ± 2.32
− 0.67 ± 2.19

InstepShowering
Short bathing
Long bathing

28.82 ± 3.76
28.36 ± 3.54
27.90 ± 3.73

30.17 ± 1.88
31.94 ± 1.29**
32.38 ± 1.33**

30.43 ± 3.44
31.86 ± 2.49
32.68 ± 2.35*

1.36 ± 2.70
3.58 ± 3.07**
4.48 ± 3.44**

0.26 ± 2.96
− 0.08 ± 3.03
− 0.30 ± 2.17

DPG

Showering
Short bathing
Long bathing− 5.06 ± 4.00
− 4.96 ± 3.93
− 5.27 ± 3.83

− 3.26 ± 1.62
− 1.47 ± 1.73**− 0.53 ± 1.62**,#− 3.27 ± 3.20
− 2.04 ± 2.13
− 1.26 ± 2.35*

1.80 ± 3.30
3.49 ± 3.49
4.74 ± 3.26*

0.01 ± 3.06
− 0.57 ± 3.16
− 0.73 ± 2.46

DPG: distal (instep) to proximal (chest) skin temperature gradient. A before bathing, B after bathing, C before sleep
B-A means B minus A, representing the change from before bathing to after bathing
C-B means C minus B, representing the change from after bathing to before sleep*

Significant difference from showering condition, p < 0.05**#

Significant difference from showering condition, p < 0.01

Significant difference from short bathing condition, p < 0.05##

Significant difference from short bathing condition, p < 0.01

Fig. 4 Thermal sensations for the whole body, back of the hand,and feet before going to sleep. Squares, triangles, and circles indicate
showering and short and long bathing conditions, respectively.
*Significant difference from showering condition, p < 0.05

Thermal sensation before sleep

Figure 4 indicates the results from thermal sensations
for the whole body, hands, and feet before going to sleep.
No differences in thermal sensation at the hands and feetwere seen among conditions. However, thermal sensation for the whole body was perceived as higher with longbathing than with showering (p = 0.045).

Discussion
From the results for SL as a sleep index, long bathing
showed a significantly shorter time required to fall asleep
compared to showering (Fig. 1). In addition, long bathing
was significantly superior to showering and short bathingin terms of sleep onset items in Factor II (initiation andmaintenance of sleep) of the OSA-MA (Fig. 3), and subjective evaluation (Fig. 2). Previous studies [13, 33] haveshown that lowering core temperature before sleep leads
to shortening the time required to fall asleep. In fact,core temperature was decreased with long bathing compared to other conditions from after bathing to before
sleep (Table 1). Bathing before going to bed is thoughtto raise the core temperature, which increases peripheral
blood flow and skin temperature at the hands and feet,thus promoting heat dissipation and finally a more rapid
decrease in core temperature. The increase in core temperature produced by bathing is thought to be effective inpromoting a greater decrease in core temperature, subsequently shortening SL.
On the other hand, the decrease in core temperatureis the result of heat loss and is not the main cause of
induced drowsiness, but rather a higher DPG promotes
sleep onset [32, 34]. Tai et al. [11] showed similar results
in a longitudinal study of 1094 elderly individuals (meanage, 72.0 years). They found that bathing before bed wasMaeda et al. Journal of Physiological Anthropology(2023) 42:20

significantly associated with shorter actigraphic/selfreported SL and higher DPG. In the present study, DPG
before sleep was significantly higher with long bathingcompared to showering because peripheral skin temperature (at the instep) was increased relative to trunkskin temperature (chest) (Table 1). To increase DPG, notonly a temporary rise in skin temperature after bathing,
but also a moderate rise in core temperature as with longbathing causes vasodilation as a thermoregulatory reaction, releasing heat from the periphery. To promote anincrease in DPG and thus shorten SL, not only warming
the periphery but also raising core temperature to thelevel observed with long bathing in this study appears
effective.
In addition, cold hands and feet before sleep induce toprolong SL [2]. SL is reportedly increased for vasospastic
individuals with low skin temperature in the extremities
and a tendency to feel cold compared to healthy controls.
This is attributed to inefficient heat dissipation from theperiphery, preventing an adequate drop in core temperature and a smooth transition to sleep.
These findings suggest that the magnitude of theincrease in deep body temperature associated with bathing, such as with long bathing, is closely associated withthe ease of falling asleep.
For sleep maintenance items in Factor II of the OSAMA, long bathing showed significantly higher value thanshowering (Fig. 3b). Previous studies have reported that
passive body heating including bathing shortened WASO
[6, 10] and increased slow-wave sleep, which is related tosleep depth [6]. In the OSA-MA, detailed question items
related to SM include depth of sleep and waking in the
middle of the night. The results of such studies support
our results for SM with long bathing. This suggested
that greater increases in core body temperature induced
improvements in sleep maintenance.
In subjective evaluations, sleep quality was significantly
better with long bathing than with showering or shortbathing (Fig. 2). Yasuda et al. [19] reported that changing
bathing habits from showering to bathing in a bathtub(at 41 °C for 10 min) improved sleep quality, particularly
in terms of the feelings of deep sleep and recovery fromfatigue. Dorsey et al. [5, 6] also reported that bathing at
40–40.5 °C for 30 min improved subjective sleep quality
in women with insomnia, specifically increasing “depthof sleep” and decreasing “restlessness”. Our finding for
OSA-MA Factor II in the present study supported thoseprevious studies. Subjective sleep quality, which is evaluated the next morning, would be influenced by not only
depth of sleep, but also the above-mentioned improvements in SM and falling asleep. Under the conditionsof this study, the increase in core temperature with long

Page 8 of 10

bathing was suggested to offer the best improvements inquality of sleep among the conditions tested.
From comparing short and long bathing, differences in
the rise in core temperature due to differences in bathingtime led to differences in subjective evaluations of sleep.
Under the condition of long bathing with a large increasein core temperature, vasodilation was induced, DPG
increased, and heat dissipation was promoted, resulting
in a large decrease in core temperature. In fact, with longbathing, core temperature was higher after bathing, anddecreased significantly from after bathing to before sleep(Table 1).
Matsumoto et al. [35] showed that increase in sublingual temperature with bathing was twice the increase inrectal temperature. Therefore, considering the increasein sublingual temperature with long bathing in this study(0.91 °C), the increase in rectal temperature was speculated to be close to the target increase of 0.5 °C. Previous studies have reported that bathing to the mid-thorax
level in 41 °C water for 1.5 h increased rectal temperatureby 1.8 ºC [3], and induced delayed sleep onset, disturbed
sleep, and frequent awakenings [7]. Although a rise inrectal temperature associated with bathing is necessary
to have a positive effect on sleep, an optimal range seems
to exist, because excessive rises in rectal temperatureadversely affects sleep.
Since this study was conducted in the homes of participants, some limitations must be considered.
First, rectal temperature was not able to be measured
as the core temperature, although the bathing time wasset based on a predicted rectal temperature change [23].
Since sublingual and rectal temperature responses tobathing differ [35], measurement of rectal temperature
may be necessary. Second, we could not continuously
measure body temperatures or autonomic nervous indices. Clarification of changes in these indices would help
us understand the findings of this study. Evaluation of
sleep stages by polysomnography would similarly deepen
the understanding of the effects on sleep of increasing
body temperature by bathing. Finally, the ages of participants were broad and included participants of both sexes
and in each menstrual stage, which may mask differencesin the effects between subgroups. Furthermore, it is necessary to evaluate the effect of lifestyle and preference forbathing duration on sleep and its individual variations.

Conclusions
In this study, field experiments on bathing and sleeping
were conducted with participants in a wide range of age
groups from young to middle-aged (20 s to 50 s) in actual
spaces. Our results showed that bathing 1.5–2 h before
bedtime with a relatively large rise in core temperature
was more beneficial for falling asleep and sleep quality

Maeda et al. Journal of Physiological Anthropology(2023) 42:20

than bathing with a small rise in core temperature. This
study is the first to verify the relationship between the
degrees of increase in core temperature associated withbathing and sleep in actual living conditions.
Abbreviations
ANOVA	Analysis of variance
DPG	Distal-to-Proximal skin temperature Gradient
OSA-MA	Oguri-Shirakawa-Azumi Sleep Inventory, Middle-aged and AgedversionSE	Sleep efficiency
SL	Sleep onset latency
SM	Sleep maintenance
TIB	Time in bed
TST	Total sleep time
WASO	Wake after sleep onset
Acknowledgements
We are grateful to all participants for their participation.
Authors’ contributions
All authors contributed to the conception and design of the study. HK and TN
collected the data. TM and HK analyzed data and drafted the original manuscript, and then SH and TN revised the manuscript. All authors have read andagreed to the final version of the manuscript.
Funding
Not applicable.
Availability of data and materials
All relevant data are resented within the paper.

Page 9 of 10

5.

6.

7.
8.
9.
10.

11.

12.
13.

14.

Declarations
Ethics approval and consent to participate
In accordance with the principles laid out in the Declaration of Helsinki, allparticipants provided written informed consent after being provided withinformation about the purposes, procedures, and possible risks of this study.
This study was approved by the ethics committee of the Noritz Corporation(approval no. 209).

15.

Consent for publication
Not applicable.

17.

16.

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

18.

Received: 31 March 2023 Accepted: 22 August 2023

19.

20.
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