Physiological Effects of Far-Infrared-Emitting Garments on Sleep, Thermoregulation, and Autonomic Function Assessed Using Wearable Sensors

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This manuscript explores the effects of far-infrared–emitting garments on sleep and thermoregulation using a randomized, double-blind, placebo-controlled crossover design and wearable sensors, which is highly relevant to the journal’s scope. The study is well organized, and the overall methodology is clearly described. The findings — including lower tympanic temperature, reduced sweating during mid-sleep, and a higher proportion of REM sleep — provide useful insights into FIR-textile impacts on sleep physiology.

However, the sample consists only of healthy young males, which limits generalizability and should be acknowledged more explicitly. The physiological changes shown are statistically significant but modest, so the authors are encouraged to better clarify the practical significance of these results. The interpretation of HRV outcomes should also be tempered, considering the debated meaning of LF power. Additional discussion on garment material characteristics and potential alternative explanations for sweating reduction would strengthen the study.

In addition, relevant publications can be cited in the introduction section to enhance the understanding of trends of this study.

1. Milić, D. Marinković and Ž. Ćojbašić , Facta Univ. Ser. Mech. Eng. (2023), https://doi.org/10.22190/FUME050123059M.

2. Ha H, Qaiser N, Yun TG, Cheong JY, Lim S, Hwang B (2023) Sensing mechanism and application of mechanical strain sensor: a mini-review. Facta Univ Ser Mech Eng 21(4):751–772

Overall, the paper is well written and contributes valuable data to the field, but addressing the points above will improve clarity and impact.

Author Response

Response to Reviewer 1

This manuscript explores the effects of far-infrared–emitting garments on sleep and thermoregulation using a randomized, double-blind, placebo-controlled crossover design and wearable sensors, which is highly relevant to the journal’s scope. The study is well organized, and the overall methodology is clearly described. The findings — including lower tympanic temperature, reduced sweating during mid-sleep, and a higher proportion of REM sleep — provide useful insights into FIR-textile impacts on sleep physiology.

However, the sample consists only of healthy young males, which limits generalizability and should be acknowledged more explicitly. The physiological changes shown are statistically significant but modest, so the authors are encouraged to better clarify the practical significance of these results. The interpretation of HRV outcomes should also be tempered, considering the debated meaning of LF power. Additional discussion on garment material characteristics and potential alternative explanations for sweating reduction would strengthen the study.

In addition, relevant publications can be cited in the introduction section to enhance the understanding of trends of this study.

1. Milić, D. Marinković and Ž. Ćojbašić , Facta Univ. Ser. Mech. Eng. (2023), https://doi.org/10.22190/FUME050123059M.
2. Ha H, Qaiser N, Yun TG, Cheong JY, Lim S, Hwang B (2023) Sensing mechanism and application of mechanical strain sensor: a mini-review. Facta Univ Ser Mech Eng 21(4):751–772

Overall, the paper is well written and contributes valuable data to the field, but addressing the points above will improve clarity and impact.

 

General Response:
We sincerely thank the reviewer for their careful evaluation and constructive comments. We are grateful for the positive assessment of our study design, methodology, and the relevance of our work to the journal’s scope. We have revised the manuscript to address all points raised. Below, we respond point by point.

 

Comment 1
“The sample consists only of healthy young males, which limits generalizability and should be acknowledged more explicitly.”

Response:
We fully agree that the homogeneity of our sample limits the generalizability of the findings. In particular, thermoregulatory capacity, sweating thresholds, and sleep architecture can differ by sex, age, and the presence of sleep or thermoregulatory impairments. We have therefore made this limitation more explicit and emphasized the need to examine more diverse populations in future work.

Changes in the manuscript:
We revised the Limitations paragraph in the Discussion as follows:
“Several limitations warrant consideration. The sample consisted of healthy young men, and results may not generalize to women, older adults, or individuals with sleep or thermoregulatory impairments.”

We also added that future studies should evaluate diverse populations and contexts with greater thermal strain or repeated use.

 

Comment 2
“The physiological changes shown are statistically significant but modest, so the authors are encouraged to better clarify the practical significance of these results.”

Response:
We appreciate this important point. We agree that the observed effects are modest and should not be overinterpreted. At the same time, even small shifts in thermal load and REM sleep proportion may be meaningful in specific contexts (e.g., suboptimal sleep environments or populations sensitive to thermal disturbance). We have revised the Discussion and Conclusions to clearly frame our findings as evidence of physiological feasibility and to clarify their potential, but limited, practical implications.

Changes in the manuscript:

1. In the REM-related Discussion paragraph, we now state:

“Because total sleep time did not change, this effect likely reflects a redistribution of sleep stages rather than an absolute extension of REM. Although the magnitude of change was modest, even small thermal adjustments can influence REM stability in individuals or environments sensitive to thermal disturbances. FIR textiles may thus offer a passive means of enhancing microclimate stability during sleep.” 

2. Later in the Discussion, we explicitly temper the interpretation:

“However, the present findings are modest in magnitude and should be interpreted as physiological feasibility, rather than clinical efficacy.”

3. In the Conclusions, we similarly state:

“While the physiological effects observed were modest and limited to healthy young men, this approach provides a basis for evidence-based development of functional sleepwear, particularly in contexts where thermoregulation is challenged.”

These revisions clarify that the primary contribution of the study is mechanistic and feasibility-oriented, and that any practical applications should be considered preliminary.

 

Comment 3
“The interpretation of HRV outcomes should also be tempered, considering the debated meaning of LF power.”

Response:
We agree that LF power should not be treated as a simple index of sympathetic activation, and we appreciate the suggestion to moderate our interpretation. In the revised manuscript, we emphasize that LF reflects a mixture of baroreflex-related and broader autonomic modulation and that its physiological meaning remains debated. We then explicitly interpret the observed LF increase as a transient thermoregulatory adjustment rather than sympathetic arousal.

Changes in the manuscript:

1. In the HRV Discussion section, we added:

“Given that LF reflects a mixture of baroreflex activity and broader autonomic modulation, its physiological meaning remains debated. Therefore, the early-night rise in LF observed here should not be taken as evidence of heightened sympathetic activation but instead as a transient thermoregulatory adjustment.”

2. We further clarified the pattern of autonomic responses:

“The transient increase in LF power during early sleep may reflect baroreflex-mediated cardiovascular adjustments associated with enhanced heat dissipation, rather than sympathetic arousal or stress-related activation. The absence of changes in HF and RMSSD further supports a stable parasympathetic tone and suggests that autonomic responses were physiologically adaptive and non-alerting.”

3. To support this interpretation, we also added a reference addressing the limitations of LF/HF as a sympatho-vagal index and work on heat stress and HRV (Billman, 2013; Flouris et al., 2014) in the reference list.

Comment 4
“Additional discussion on garment material characteristics and potential alternative explanations for sweating reduction would strengthen the study.”

Response:
We agree that differences in textile properties beyond FIR emission may contribute to sweating patterns. Although FIR and control garments were matched for color, fabric weight, thickness, and knit structure, we did not directly measure thermal resistance or evaporative capacity. We have now expanded the Discussion to acknowledge potential contributions of moisture transport and heat flux and to highlight the need for combined textile-level and physiological measurements in future studies.

Changes in the manuscript:
In the thermoregulation section of the Discussion, we added:

“Although we matched FIR and control garments for color, fabric weight, thickness, and knit structure, we did not directly quantify textile thermal resistance or evaporative capacity. Differences in moisture transport or local heat flux may therefore also contribute to the observed sweating patterns. These findings highlight the importance of combining textile-level measurements with physiological monitoring in future work.”

In the Limitations paragraph, we also note that direct textile properties (e.g., in situ thermal resistance and emissivity) were not measured and should be included in future research.

 

Comment 5
“Relevant publications can be cited in the introduction section to enhance the understanding of trends of this study.” (with suggested references to Milić et al., 2023 and Ha et al., 2023)

Response:
We thank the reviewer for these helpful suggestions. We have incorporated the recommended references to better situate our work within broader trends in textile engineering and mechanical strain–sensor research. Although the present study focuses on sleep physiology rather than detailed structural mechanics, these citations strengthen the contextual background regarding functional and sensor-integrated textiles.

Changes in the manuscript:
In the Introduction, we added:

“Furthermore, advances in stretchable and piezoelectric textile-based sensors have demonstrated that wearable fabrics can integrate mechanical strain–sensor functionalities, potentially enabling future multi-modal monitoring beyond thermal indices [9,10].”

The corresponding references are:

9. Ha, H.; Qaiser, N.; Yun, T.G.; Cheong, J.Y.; Lim, S.; Hwang, B. Sensing Mechanism and Application of Mechanical Strain Sensors: A Mini-Review. Facta Univ. Ser. Mech. Eng. 2023, 21, 751–772.
10. Milić, D.; Marinković, Ž.; Ćojbašić, Ž. Geometrically Nonlinear Analysis of Piezoelectric Active Laminated Shells by Means of Isogeometric FE Formulation. Facta Univ. Ser. Mech. Eng. 2023, 23, 387–405.

We are grateful to Reviewer 1 for these thoughtful comments, which have helped us to clarify the limitations, temper our interpretations, and strengthen the contextualization of our work. We believe that the revised manuscript has been significantly improved as a result.

Reviewer 2 Report

Comments and Suggestions for Authors

This article demonstrates the impact of a new type of far-infrared emitting clothing on the sleep of subjects, using many physiological sensors to monitor and analyze their sleep quality during testing. This article focuses on the comparative analysis of data between the test group and the control group, which has certain research reference value for relevant researchers. However, the scientific research depth of this article is insufficient, and many key scientific issues have not been explored, which requires more data support. Here are some examples to illustrate.

 

The working principle of far-infrared emitting clothing should be explained in detail, especially in the introduction section, which should have a more specific introduction and analysis.

 

2.2. FIR and Control Garments

The FIR-emitting garment was constructed from polyester fibers embedded with ceramic nanoparticles known to emit FIR wavelengths in the 5–20 μm range, verified by manufacturer spectroscopic analysis.

 

This paragraph lacks many necessary technical details, such as:

• The material composition of these garments should be specifically described, including the ratio of polyester fibers and ceramic nanoparticles;
• What is the basis for selecting fiber materials for making clothing? Why use polyester fiber instead of cotton or other materials?
• Have you made composite materials of different types of fiber materials and ceramic nanoparticles? Do you have any relevant fiber function testing data and analysis?
• What is the manufacturing process of these fibers?
• What are the sizes and styles of these clothes? It's best to have pictures to illustrate;
• The textile fibers of ceramic nanoparticle materials can emit infrared spectra, and its principle should be described in detail in the introduction section, which should provide spectral analysis data.

 

 

2.3. Participants

Fifteen healthy male university students (age 20–21 years) participated in the study.

 

As an article discussing scientific research, it is imperative to conduct tests on clothing materials and functions before conducting human volunteer tests. Subsequently, animal experiments should be carried out to validate the effectiveness of the clothing. Only after these tests have proven the efficacy of the clothing can human tests be conducted. However, this article lacks these essential steps.

• In addition, is the testing arrangement serial testing or parallel testing?
• How many subjects can be tested in one batch?
• How many sets of equipment are being used simultaneously?
• What is the gender ratio of the subjects?

 

2.6.1. EEG-Based Sleep Staging

Electroencephalogram (EEG) signals were recorded using a portable sleep-monitoring system (Insomnograf K2; S’UIMIN Inc., Japan), which uses five electrodes placed on the forehead and neck to acquire EEG, EMG, and EOG with minimal invasiveness.

 

• Merely describing these words is not enough, it is recommended to include a schematic diagram.
• Does the placement of this wired collection electrode in the head and neck position affect the sleep of the tester? Have you considered choosing a wireless solution to reduce interference to the subjects.
• Suggest presenting the collected signals EEG, EMG, and EOG in waveform form.
• Analyze the collected signals, including both valid and invalid data.

 

2.6.2. Tympanic Temperature, Sweating Rate, Skin Temperature, and Skin Humidity

TMT, sweating rate, skin temperature, and skin humidity were continuously monitored using the BL100 wearable device (Technonext, Japan).

 

This part should be accompanied by a physical image and marked with necessary measurement details, including:

• The method and location of fixing the sensor;
• The volume size of the sensor;
• The working mode of the equipment;
• Is the device communication wired or wireless?
• Display the signals collected by the device or showcase the user interface of the device;
• Does the sensor of the device interfere with the subjects?

 

2.6.3. Heart Rate Variability (HRV)

HRV was assessed using the Actiheart 5 device (CamNtech, UK), recording single-lead ECG at 128 Hz. R-peaks were detected using the device’s validated QRS algorithm [19].

 

ECG acquisition equipment also lacks details:

• Does the device adopt wired or wireless data collection method?
• Is the electrode position collected in the chest lead or the limb lead?
• A schematic diagram of the subjects wearing the device should be displayed;
• Will the simultaneous connection of so many devices, including EEG electrodes, ECG electrodes, and other sensors mentioned in the article, interfere with the sleep quality of the same subject?

 

Other minor issues in the article:

• When professional terms first appear, their full names should be given. Even in the abstract section, attention should be paid. For example, LF in line 37 is not defined;
• Professional terms should not be defined repeatedly. For example, Far intrinsic (FIR) has been defined multiple times in the manuscript. Duplicate definitions appear on lines 67, 89, and 314. There are many other similar issues.

Author Response

Response to Reviewer 2

General Response
We sincerely thank the reviewer for their careful evaluation and constructive comments. We appreciate the reviewer’s interest in the textile and sensing technologies underlying this study. Several points relate to material engineering, manufacturing processes, or preclinical testing, which are beyond the primary scope of a human sleep physiology study; nevertheless, we have revised the manuscript to clarify the working principles of the FIR garments, the rationale for material choices, and the details of our measurement systems and procedures. Below, we respond point by point.

 Comment 1

“The working principle of far-infrared emitting clothing should be explained in detail.”

Response:

We agree that a concise explanation of the working principle is important for readers who are less familiar with FIR textiles. We have therefore expanded the Introduction to describe how ceramic nanoparticles embedded in synthetic fibers enhance emissivity in the far-infrared band and how this relates to human thermal radiation.

Changes in the manuscript:

In the Introduction, we now state:

“Far-infrared (FIR)–emitting textiles incorporate ceramic nanoparticles that enhance emissivity in the 5–20 μm band, corresponding to the human body’s peak thermal radiation spectrum [16–18]. FIR radiation is absorbed by superficial tissues and re-emitted as thermal energy, which may promote peripheral vasodilation and radiative heat dissipation [17–19].”

This passage is supported by references on FIR materials and biomedical applications.

Comment 2

“Material composition, fiber ratios, manufacturing process, etc., should be specified.”

Response:

We appreciate this request. However, detailed fiber ratios, nanoparticle loading, and fabrication parameters are proprietary to the garment manufacturer and were not disclosed to the investigators. From the perspective of a human physiological evaluation, the key requirement is to verify that the textile exhibits FIR emission in the intended band, which was confirmed by independent spectroscopic testing. We have clarified this limitation in the Methods and in the Limitations section.

Changes in the manuscript:

In the Methods (Section 2.3, FIR and Control Garments), we now write:

“The FIR garment consisted of polyester fibers embedded with ceramic nanoparticles emitting in the 5–20 μm band [17–19]. The control garment was visually identical and matched for fabric weight, thickness, and knit structure, differing only in the absence of FIR-emitting components. Manufacturer-provided spectroscopic testing confirmed FIR emissive properties. Fiber composition details were not disclosed due to proprietary restrictions but are not required for physiological evaluation.”

In the Limitations, we also note that in situ textile properties (e.g., thermal resistance, emissivity) were not directly measured and should be included in future work.

 

Comment 3

“Why polyester? Why not cotton or other materials?”

Response:

We agree that the choice of base fiber is relevant. Polyester was selected because ceramic nanoparticles can be stably embedded into synthetic polymers during extrusion, enabling durable FIR emissive properties. In contrast, natural fibers such as cotton cannot be processed in this way; FIR functionality would require surface coatings that are less durable to washing and mechanical stress. We have added this rationale to the Methods.

Changes in the manuscript:

In Section 2.3, we now add:

“Polyester was selected as the base fiber because ceramic nanoparticles can be uniformly embedded within the polymer matrix during extrusion, conferring durable FIR emissivity. Natural fibers such as cotton cannot be processed in this manner and would require surface coatings, which are generally less wash-resistant.”

 

Comment 4

“Animal experiments should precede human trials.”

Response:

We respectfully disagree that animal experiments are necessary or appropriate in this context. The primary objective of the present study is to evaluate the thermoregulatory, autonomic, and sleep-related effects of wearable sleep garments in humans. The relevant physiological processes—sleep architecture, sweating patterns, perceived comfort, and the skin–textile microclimate—are fundamentally human-centered and depend on garment fit and behavioral factors that cannot be meaningfully modeled in animals. Human volunteer trials under controlled environmental conditions are therefore the standard and ethically appropriate approach for evaluating sleepwear.

We now explicitly state that this work represents a physiological feasibility study in healthy adults and does not involve pharmacological or invasive interventions.

 

Changes in the manuscript:

In the Introduction/Discussion, we added:

“Because the performance of sleepwear depends on human thermoregulation, sweating, and sleep architecture, animal models are not appropriate for evaluating FIR garments. Controlled human volunteer studies represent the most relevant and ethically appropriate approach for physiological feasibility testing of sleep-related textiles.”

 

Comment 5

“Please specify serial vs. parallel testing, gender ratio, batch size, number of equipment sets.”

Response:

We thank the reviewer for this suggestion. We have clarified that all participants were male, that sessions were conducted individually on separate nights, and that dedicated equipment sets were used such that only one participant was recorded per night, avoiding any interference or resource sharing.

 

Changes in the manuscript:

In the Participants and Procedures sections, we now state explicitly:

“Fifteen healthy male university students (20–21 years) participated (15/15 male). Testing was conducted individually, with one participant recorded per night, using dedicated EEG, ECG, and thermoregulatory sensor sets for each session. Overnight sessions were scheduled on separate nights with a ≥3-day washout period between conditions.”

 

Comment 6

“Schematic diagrams should be added for EEG, ECG, and wearable sensors.”

Response:

We agree that schematic illustrations of device placement improve clarity. We have added a supplementary figure showing the configuration and placement of the EEG device, ECG leads, and thermoregulatory sensors.

Changes in the manuscript:

We added the following statement in the Methods:

“Sensor placement for EEG, ECG, tympanic temperature, sweating, skin temperature, and humidity is illustrated in Supplementary Figure S1.”

The supplementary figure provides a clear schematic of device positions without revealing proprietary device designs.

 

Comment 7

“Does wired EEG/ECG interfere with sleep?”

Response:

We appreciate this concern. The EEG and ECG systems used in this study (Insomnograf K2 and Actiheart 5) are validated for overnight monitoring and have been shown to exert negligible effects on sleep architecture and comfort in prior research. Both systems are specifically designed for home or laboratory sleep studies and use lightweight, minimally obtrusive leads or sensor modules. We have clarified this in the Methods and added appropriate citations.

 Changes in the manuscript:

In Section 2.5 (Physiological Signal Acquisitions), we now include:

“Sleep staging was performed using a validated in-home EEG device (Insomnograf K2; S’UIMIN Inc.), which has demonstrated >95% agreement with polysomnography and negligible impact on sleep architecture [13]. Autonomic activity was recorded using Actiheart 5, a combined ECG and movement sensor validated for overnight monitoring [15]. Both devices are lightweight and designed to minimize discomfort, and prior work indicates that they do not meaningfully alter sleep continuity or architecture.”

In the Limitations and Discussion, we also note that subjective sleep quality did not differ between conditions, further suggesting that instrumentation did not introduce noticeable sleep disruption.

 

Comment 8

“Photos of sensors, UI, and volume dimensions should be added.”

Response:

We agree that information about device size and attachment methods is useful. We have added basic descriptions of sensor dimensions and fixation methods in the Methods. However, providing photographs of proprietary hardware and user interfaces requires manufacturer permission and is not typically mandatory for sleep physiology studies. We believe the added schematic (Supplementary Figure S1), together with the textual descriptions, offers sufficient transparency regarding the experimental setup.

Changes in the manuscript:

In Section 2.5, we now describe:

“The Insomnograf K2 EEG unit and Actiheart 5 module are compact, lightweight devices (approximately 30–50 g), attached using medical-grade adhesive patches. The BL100 thermoregulatory sensors (Technonext) were affixed to the ear canal (TMT), chest, and forearm using soft adhesive holders, allowing participants to change body position freely during sleep.”

We also refer readers to Supplementary Figure S1 for visual placement.

 

Comment 9

“LF not defined at first use; FIR defined repeatedly.”

Response:

We thank the reviewer for pointing out these editorial inconsistencies. In the revised manuscript, we have ensured that all abbreviations are defined at first use and that repeated, unnecessary re-definitions are removed.

Changes in the manuscript:

• In the Abstract and Methods, “low-frequency power (LF)” and “high-frequency power (HF)” are defined at their first appearance.
• “Far-infrared (FIR)” is defined once in the Abstract and at its first use in the main text; redundant re-definitions have been removed.

Once again, we are grateful to Reviewer 2 for these thoughtful and technically oriented comments. They have helped us clarify methodological details, better justify our material choices, and improve the transparency of our experimental setup. We believe the revised manuscript is clearer and more informative as a result.

Reviewer 3 Report

Comments and Suggestions for Authors

In this paper, the authors studied multimodal wearable sensors and analyzed the physiological effects of far-infrared emitting clothing on sleep, body temperature regulation, and autonomous functions. The results show that far-infrared clothing can lower the temperature of the eardrum, reduce mid-sleep sweating, increase the proportion of rapid eye movement sleep, and simultaneously enhance the low-frequency power of early sleep, providing a new textile intervention method for optimizing the sleep thermal environment. However, the research sample was limited to healthy young men, which has certain limitations. Moreover, there was no in-depth analysis of the differences in body temperature regulation at different sleep stages. Therefore, the universality of the conclusion needs further verification. In conclusion, the authors provide a good example of the application of wearable sensing technology in the functional evaluation of textiles.

1. The study only included healthy males aged 20-21, excluding females and middle-aged or elderly populations. What specific impact might this sample selection have on the generalizability of the research conclusions?

2. The ceramic nanoparticles in the far-infrared clothing emit infrared radiation at wavelengths of 5–20 μm. Would adjusting this wavelength range change its effects on sleep and thermoregulation?

3. The study observed a significant increase in the proportion of REM sleep, but total sleep time did not change. What are the specific physiological mechanisms behind this phenomenon?

4. Far-infrared clothing only reduced sweating during mid-sleep, with no significant effects in the early or late sleep stages. What causes this time-specific difference?

5. The control clothing and far-infrared clothing differ only in fiber composition. Is there any difference in long-term subjective experiences, such as breathability and comfort, between the two?

6. Far-infrared clothing led to increased low-frequency power during early sleep. Why did high-frequency power and the standard deviation of heart rate variability not change?

7. In the practical promotion of far-infrared clothing from this study, how can the stability of ceramic nanoparticles be balanced with the wash durability of the clothing?

Author Response

Response to Reviewer 3

We sincerely thank the reviewer for their thoughtful and constructive comments. We appreciate the positive evaluation of our study as an example of applying multimodal wearable sensors to the functional evaluation of far-infrared (FIR) textiles, and we have carefully revised the manuscript to address each of the points raised. Below, we respond to all comments point by point.

 

General Comment

“In this paper, the authors studied multimodal wearable sensors and analyzed the physiological effects of far-infrared emitting clothing on sleep, body temperature regulation, and autonomous functions… In conclusion, the authors provide a good example of the application of wearable sensing technology in the functional evaluation of textiles.”

 

Response:

We are grateful for the reviewer’s positive overall assessment and for recognizing the relevance of wearable sensing in textile evaluation. In response to the reviewer’s specific questions, we have clarified the limitations of our sample, expanded discussion of stage-specific thermoregulation, provided more detailed mechanistic interpretations for REM sleep and heart rate variability (HRV) findings, and added comments on long-term comfort and durability of FIR garments. All changes are incorporated in the revised manuscript as detailed below.

 

Comment 1

“The study only included healthy males aged 20–21, excluding females and middle-aged or elderly populations. What specific impact might this sample selection have on the generalizability of the research conclusions?”

Response:

We fully agree that restricting the sample to healthy young men limits the generalizability of our conclusions. Thermoregulatory capacity, sweating thresholds, skin blood flow, and sleep architecture differ by sex, age, and hormonal status. For example, women show menstrual-phase-dependent changes in core body temperature and REM expression, and older adults exhibit reduced sweating capacity and a blunted nocturnal decline in core temperature. As a result, FIR-induced thermal and autonomic responses observed in our study may not directly extend to women, older adults, or individuals with sleep or thermoregulatory impairments. We have revised the Discussion to more clearly state this limitation and to emphasize the need for future studies in diverse populations.

 

Changes in the manuscript:

In the Limitations section of the Discussion, we now state:

“Several limitations warrant consideration. The sample consisted of healthy young men, and results may not generalize to women, older adults, or individuals with sleep or thermoregulatory impairments.”

We further note that future studies should include more diverse populations to examine whether FIR-induced thermal and autonomic responses differ across demographic and clinical groups.

 

 

Comment 2

“The ceramic nanoparticles in the far-infrared clothing emit infrared radiation at wavelengths of 5–20 μm. Would adjusting this wavelength range change its effects on sleep and thermoregulation?”

Response:

The 5–20 μm emission band of FIR textiles closely overlaps with the human body’s thermal radiation spectrum (peaking around 9–10 μm). Ceramic nanoparticles embedded in the fibers increase emissivity across this band, enhancing radiative heat exchange between the body and garment. At present, there is no empirical evidence that tailoring the emission spectrum within this 5–20 μm range produces qualitatively different physiological outcomes during sleep. It is likely that overall emissivity across the relevant band, rather than precise spectral tuning within narrow sub-ranges, is the dominant determinant of physiological effects. Nonetheless, exploring whether different FIR emission profiles might yield stage-specific or magnitude-dependent effects is an interesting topic for future materials research.

Changes in the manuscript:

In the Methods section describing FIR garments and in the Discussion, we added:

“FIR textiles typically emit in the 5–20 μm range, matching the human body’s thermal radiation spectrum. Current evidence suggests that overall emissivity within this band, rather than specific sub-band wavelengths, primarily governs physiological effects. Whether tailoring emission spectra would yield stage-specific or magnitude-dependent responses during sleep remains an open question.”

 

Comment 3

“The study observed a significant increase in the proportion of REM sleep, but total sleep time did not change. What are the specific physiological mechanisms behind this phenomenon?”

Response:

We interpret the increased proportion of REM sleep as a redistribution of sleep stages under a more thermally favorable microenvironment, rather than an absolute extension of REM duration. REM sleep is highly sensitive to thermal conditions and is facilitated when thermoregulatory demands are minimal (i.e., near thermoneutrality), with reduced need for active vasomotor or sweating responses. In our study, FIR garments lowered tympanic temperature and reduced the evaporative burden during mid-sleep, a period when REM density normally increases. Under such conditions, REM episodes may be more easily initiated and maintained.

Because total sleep time did not differ between conditions, the increased REM proportion likely reflects a shift in stage allocation rather than an absolute increase in REM minutes. We have clarified this mechanism in the revised Discussion and supported it with additional references on REM and thermoregulation.

 

Changes in the manuscript:

In the Discussion section on sleep architecture, we now state:

“REM expression depends on a state of thermoneutrality, in which active thermoregulatory defenses are minimized. By lowering heat load, FIR garments may reduce the need for evaporative cooling and peripheral vasomotor adjustments, thereby facilitating REM initiation and maintenance. Because total sleep time did not change, this effect likely reflects a redistribution of sleep stages rather than an absolute extension of REM. Although the magnitude of change was modest, even small thermal adjustments can influence REM stability in individuals or environments sensitive to thermal disturbances. FIR textiles may thus offer a passive means of enhancing microclimate stability during sleep.”

We also added references on the coupling between REM sleep and thermoregulatory suspension in the reference list.

 

Comment 4

“Far-infrared clothing only reduced sweating during mid-sleep, with no significant effects in the early or late sleep stages. What causes this time-specific difference?”

Response:

We agree that this time-specific pattern is important to interpret. Mid-sleep typically coincides with the nocturnal minimum of core body temperature, at which time evaporative cooling (sweating) becomes a dominant thermoeffector mechanism for fine-tuning heat balance. In contrast, during early sleep, heat loss is primarily supported by non-evaporative mechanisms such as peripheral vasodilation and increased distal skin temperature, and late sleep is characterized by a rise in core temperature toward awakening.

In our study, FIR garments lowered tympanic temperature, thereby reducing the need for evaporative heat loss during the mid-sleep window when sweating is physiologically most relevant. This thermophysiological profile explains why sweating was reduced specifically in mid-sleep but not in early or late sleep, where other thermoregulatory pathways predominate.

Changes in the manuscript:

In the thermoregulation section of the Discussion, we added:

“Sweating decreased only during mid-sleep, the interval when evaporative heat loss becomes the primary thermoregulatory mechanism. FIR-induced reductions in tympanic temperature likely reduced the evaporative burden specifically during this phase, whereas early and late sleep rely more heavily on non-evaporative heat exchanges.”

We also note in the Limitations that more detailed stage-specific analyses and additional measures (e.g., distal–proximal temperature gradients, skin blood flow) would help further elucidate these time-dependent effects.

 

Comment 5

“The control clothing and far-infrared clothing differ only in fiber composition. Is there any difference in long-term subjective experiences, such as breathability and comfort, between the two?”

Response:

We did not formally assess long-term subjective wearability or comfort. For the purposes of this single-night physiological study, FIR and control garments were matched for thickness, knit structure, and mass to minimize tactile differences, and no participant spontaneously reported differences in warmth, breathability, or comfort during the laboratory nights. However, we agree that prolonged or repeated use in daily life could reveal perceivable differences related to moisture transport, thermal perception, or fabric hand. We have added this point to the Limitations and highlighted the need for future studies that incorporate validated comfort and wearability assessments under long-term, real-world usage.

Changes in the manuscript:

In the Limitations section, we now state:

“The FIR and control garments differed in fiber composition, although they were matched for tactile properties and thickness; long-term comfort and breathability were not assessed. Prolonged or repeated use may reveal perceptible differences related to moisture transport or thermal perception, and future work should incorporate validated comfort assessments to evaluate subjective wearability.”

 

Comment 6

“Far-infrared clothing led to increased low-frequency power during early sleep. Why did high-frequency power and the standard deviation of heart rate variability not change?”

Response:

Low-frequency (LF) power reflects a mixture of baroreflex-mediated cardiovascular adjustments and broader autonomic modulation, and should not be interpreted as a pure index of sympathetic activity. The early-night increase in LF observed in the FIR condition is likely related to thermoregulatory cardiovascular adjustments to enhanced heat dissipation (e.g., changes in peripheral vascular tone and central blood volume), rather than an arousal-related sympathetic activation.

High-frequency (HF) power and time-domain indices such as RMSSD are more robust markers of cardiovagal (parasympathetic) tone. The absence of changes in HF and RMSSD indicates that parasympathetic activity remained stable, which supports the interpretation that the LF increase was a transient thermoregulatory adjustment rather than a shift in sympathovagal balance. We have clarified this interpretation and added references on the debated meaning of LF power and on HRV responses to heat stress.

Changes in the manuscript:

In the HRV Discussion section, we now write:

“The isolated increase in LF power likely reflects baroreflex-related thermal adjustments during early sleep. Stable HF and RMSSD values indicate that parasympathetic tone was not altered, consistent with a thermoregulatory rather than sympathetic arousal response. Given that LF reflects a mixture of baroreflex activity and broader autonomic modulation, its physiological meaning remains debated, and caution is warranted when interpreting LF changes in isolation.” 

We also added literature on HRV under heat stress and on the limitations of LF/HF as a sympatho-vagal index to the reference list.

 

Comment 7

“In the practical promotion of far-infrared clothing from this study, how can the stability of ceramic nanoparticles be balanced with the wash durability of the clothing?”

Response:

We appreciate this practical question. The ceramic nanoparticles in FIR textiles are embedded within the polymer matrix of synthetic fibers during extrusion, rather than applied as superficial coatings. This integration generally enhances wash durability and mechanical stability, as nanoparticles are physically entrapped in the fiber structure. Prior materials studies have reported high retention of FIR emissive properties after repeated laundering cycles.

However, long-term stability can still vary depending on particle dispersion, polymer compatibility, extrusion parameters, and subsequent fabric finishing processes. We therefore agree that standardized wash-resistance testing (e.g., ISO 6330) is essential for large-scale deployment of FIR garments. We have added a brief discussion of this point in the revised manuscript.

Changes in the manuscript:

In the Discussion’s practical implications/limitations, we added:

“Ceramic nanoparticles are embedded within the polymer matrix of the fibers, improving wash durability compared with surface-coated fabrics. Nonetheless, long-term emissive stability may vary with nanoparticle dispersion, polymer compatibility, and manufacturing parameters. Standardized wash-resistance testing will be essential for evaluating the durability of FIR garments intended for long-term practical use.”

Once again, we thank you for their insightful and constructive comments. These suggestions have helped us to clarify the physiological mechanisms, acknowledge important limitations, and better articulate the practical implications and future directions of our work. We believe the manuscript has been substantially improved as a result.

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

This article has responded to the review comments one by one, which has significantly improved the quality of the article. The only drawback is that I did not see the supplementary figure S1 and could not find its location. However, this does not affect the overall evaluation of this article.

Author Response

Reviewer 2

This article has responded to the review comments one by one, which has significantly improved the quality of the article. The only drawback is that I did not see the supplementary figure S1 and could not find its location. However, this does not affect the overall evaluation of this article.

Response:

We sincerely thank the reviewer for the careful evaluation of our manuscript and for the positive assessment of its overall quality.

Regarding the comment on Supplementary Figure S1, we have clarified its location by explicitly referring to it in the main manuscript. Specifically, we added the following sentence in Section 2.5. Physiological Signal Acquisitions:

“Sensor placement and measurement configuration were shown in Supplementary Figure S1.”

The supplementary figure is provided in the Supplementary Materials section. We believe this clarification resolves the reviewer’s concern.

Reviewer 3 Report

Comments and Suggestions for Authors

The authors have comprehensively revised the paper and replied to each comment one by one. After the technical check/text editing of the journal, the paper should be accepted.

Author Response

Reviewer 3

The authors have comprehensively revised the paper and replied to each comment one by one. After the technical check/text editing of the journal, the paper should be accepted.

 

Response:

We sincerely thank the reviewer for the careful evaluation of our revised manuscript and for the positive and encouraging comments.

As suggested, we carefully rechecked the text throughout the manuscript. In addition, to improve clarity, we explicitly referred to the supplementary material in the main text. Specifically, the following sentence was added in Section 2.5. Physiological Signal Acquisitions:

“Sensor placement and measurement configuration were shown in Supplementary Figure S1.”

We believe that this clarification, together with the previous revisions, sufficiently addresses the reviewer’s comment.