Available online at www.sciencedirect.com
ScienceDirect Procedia Engineering 121 (2015) 1126 – 1132
9th International Symposium on Heating, Ventilation and Air Conditioning (ISHVAC) and the 3rd International Conference on Building Energy and Environment (COBEE)
Investigation of Human Thermal Comfort in Sleeping Environments Based on the Effects of Bed Climate Cong Song*, Yanfeng Liu, Xiaojun Zhou, and Jiaping Liu School of Environmental and Municipal Engineering, Xi'an University of Architecture and Technology, Xi'an, China
Abstract The purpose of this study is to investigate the thermal environment while sleeping through experiment. Twelve health volunteers were selected as subjects. The indoor thermal environment and bed climate were obtained via objective measurement, and the subjective thermal perceptions were collected by questionnaires. The results showed that thermal evaluations on face perceived comfort when exposed to the indoor temperature in the range of 11.3ć to 21.9ć; it was a range of 30.3ć to 32.5ć for a comfortable bed climate which directly affected the body trunk. This may because the bed cover allowed of an isolated high bed climate temperature, and facilitated a better thermal sensation under low temperature indoor environment. The results may contribute to a more preferable thermal design for civil building, and be beneficial for energy conservation. © Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license ©2015 2015The TheAuthors. Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of ISHVACCOBEE 2015. of ISHVAC-COBEE 2015 Peer-review under responsibility of the organizing committee
Keywords: Thermal environment, Bed climate, Thermal comfort, Sleeping
1. Introduction Sleep is recognized as a phenomenon controlled by the circadian rhythms. It is generally believed that sleep is important for stabilizing the energy, supporting healthy immune system of human body, besides, it is beneficial for brain plasticity and mental health [1,2]. The thermal environment while sleeping are perceived by human beings both cutaneously stimulated and emotionally provoked. A preferable thermal environment should be within the thermal regulation range of human body and evoke the best thermal sensation.
* Corresponding author. Tel.: +86-15102987164. E-mail address:
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1877-7058 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of ISHVAC-COBEE 2015
doi:10.1016/j.proeng.2015.09.118
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With regard to the thermal environment for sleeping during night, some researchers [3-5] have performed experiments to define the optimal indoor temperature in order to reach the thermal comfort condition. However, the recommended temperature were in a large range, and the role of bed cover or quilt was underestimated. People sleep with bedcovers for most of the time. The use of bedcover allowed for the development of an isolated high bed climate temperature, which was critical for maintaining both a high level of skin blood flow and skin temperature during sleep [6]. As a result, the thermal environment which human body exposed to turned out to be the microclimate in the bed, ambient conditions exerted only indirect influence on the covered body. Moreover, the quilt coverage was not the whole body in general, that is, people would leave head outside the covers. In this case, people experienced different thermal environment for different parts of the body at the same time. How does these two thermal environments affect the thermal sensation respectively? What is the thermal comfort environment for both the body parts? To solve the problems above, experiments were conducted based on typical indoor room temperatures in Northwest China (for both heating and non-heating cases). Subjects answered detailed questionnaires to assess the bed climate, the pre-sleep and post-sleep indoor thermal environment, besides, their skin temperatures and bed climate temperatures were measured under different cases. 2. Methods 2.1. Climate chamber The experiment was conducted in the Research Center of China Building Climate and Energy Efficiency at Xi'an University of Architecture and Technology. The chamber was equipped with air conditioning and electrothermal films. The air temperature, air speed and humidity was controlled by a model KFR-26GW(26556)FNDc-3 air conditioner. The electrothermal films of model US65P250M220V were spread on the ceiling, internal wall and floor in order to control the temperature of the room internal surface. Two beds were arranged in the center of the chamber. 2.2. Subjects 12 healthy Chinese students were selected as subjects with a mean age of 23.4 ± 1.4 years, and a mean body mass index of 20.4 ± 1.5 kg/m2. Approvals for the study were obtained from every subject; related trainings including the understanding of the questionnaires, the operation of experimental devices and items requiring attention were informed to them prior to the experiment. 2.3. Devices and measurements The subjects' skin temperatures were tested by a temperature logger iButton DS1922L Thermochron. The wireless devices were attached to the skin-measuring sites.The bed climate including the temperature and humidity of the air under the cover were measured by iButton 1923 Hygrochron. The logging interval was 10 minutes. Fixed on a suitable bracket, the devices were stitched on the surface of the bedcovers. The measurement points were set over the chest and abdomen beneath the bedcover, and on the surface of mattress under the back[7-9]. The mean temperature of bed climate (MBT) could be calculated by the average temperature of the above points, that is, the upper parts temperature in bed climate. Ambient temperature and the relative humidity were measured using a self-recording thermohygrometer Thermo Recorder TR-72ui. The data were collected every 10minutes.The surrounding surfaces temperature was tested using the copper-constantan thermocouples. The mean radiant temperature was calculated by area weighted averaging. The operative temparature (to) could be obtained by the integreation of the air temperature and radiant temperature according to the calculation formula in previous study[10]. During the experiment, subjects dressed in long-sleeved pajamas during sleeping period. The thermal resistance of the bed system including the mattress, the quilt and pajamas was 3.73clo calculated according to a previous study [11].
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2.4. Questionnaires Subjects were asked to evaluate the thermal environment on waking by filling the questionnaires. Their thermal sensation included the ambient environment on the face and the bed climate on the trunk during the night. A sevenpoint scale was used for thermal sensation assessment according to the ASHRAE (2010)[12] The thermal comfort evaluation about the face and trunk were also covered, and was assessed on a five-point scale, as shown in Table 1. Table 1. Descriptors for the evaluation. a. TSV -3
-2
-1
0
1
2
3
Cold
Cool
Slightly cool
Neutral
Slightly warm
Warm
Hot
b. TCV 0
1
2
3
4
Comfort
Slightly uncomfortable
Uncomfortable
Very uncomfortable
Extremely uncomfortable
2.5. Statistical analysis For each experimental case, the mean value and standard error for the subjective evaluation and objective measurement were processed. The thermal sensation and thermal comfort votes were also fitted with linear or quadratic polynomial equations, through which subsequent analysis could be conducted. 3. Results The thermal perception about indoor climate pre-sleep and about the bed climate during sleep could be obtained through our preceding study as shown in Figure 1 and Figure 2. TSV for both the evaluations increased as the increasing of operative temperature. However, the rates of increment were obviously different, and the difference between the two items were also in an increasing trend with the rising temperature; TCV of the bed climate reflected a climbing trend for higher operative temperatures.
Fig. 1. TSV of indoor climate pre-sleep and bed climate during sleep
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Fig. 2. TCV of indoor climate pre-sleep and bed climate during sleep
A comparison between the temperature of indoor climate and bed climate could be seen in Figure 3. The range of the operative temperature was about 10.5ć, while the difference between the maximum and minimum value of MBT was 2.8ć. The operative temperature increased gradually, and were in the range of the typical cases occurred in cold weather. Though increased with the rising of operative temperature, MBT were climbing in a narrow range.
Fig. 3. Temperature of indoor climate and bed climate
The thermal evaluation about the ambient environment on the face is shown in Figure 4 and Figure 5. It could be seen that face-TSV complied with a linear change with operative temperature. According to the recommendation of ISO7730 that when TSV in the range of -0.5 to 0.5, the corresponding value of operative temperature fell in range of 15.1ć to 19.3ć. The value of TCV on face could be regressed to a quadratic polynomial equation. When operative temperature between 11.3ć and 21.9ć, people could be in comfortable state on their face.
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Fig. 4. TSV of indoor climate on human face
Fig. 5. TCV of indoor climate on human face
For the body part under the quilt, thermal sensation and thermal comfort votes were obtained from subjects' trunk perception. Since the trunk was exposed to the bed climate, a linear fit of trunk -TSV with MBT was performed as shown in Figure 6. According to the recommendation range of TSV, the proposed MBT was between 30.9ć and 31.7ć. The quadratic polynomial fit of trunk-TCV is shown in Figure 7. The proposed MBT was between 30.3ć and 32.5ć.
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Fig. 6. TSV of bed climate on body trunk
Fig. 7. TCV of bed climate on body trunk
4. Discussion The most remarkable finding of this study was that people could be gratified in sleeping environment of much lower ambient temperature with proper quilts. The bed system employed in the experiment was frequently used in ordinary occupants for cold weather. The temperature in the bed climate fluctuated in a much narrower range in comparison with that of the indoor climate, and people could be in a comfortable state on face which exposed to the ambient environment. This may because the bed cover contributes to an isolated bed climate temperature, and facilitate a better thermal sensation under low temperature indoor environment. In some previous study[13,14], the bed climate temperature was relatively low, and this may due to the different measure points and different calculation methods. Besides, the results obtained were under the described environmental conditions. According to the thermal evaluations for both the face and trunk part of the body, the proposed indoor temperature during night-time sleep could be obtained. The exposed part of the body, normally the head, perceived to be comfortable in a much wider temperature range of 11.3ć to 21.9ć. The trunk part covered by quilts subjective to the bed climate was less sensitive to the ambient environment. A comfortable condition could be reached when the mean bed climate ranged in temperature from 30.3ć to 32.5ć. Indoor temperature for civil building is designed regardless of night and daytime in general. It not only caused an
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unnecessary waste of energy, but also an discomfort on human experience. The thermal design including the indoor environment and bed climate may be optimized if further studies are conducted, thus providing a more preferable and energy saving environment for occupants' living. 5. Conclusions Bed climate plays an significant role in the thermal comfort during sleeping period, Experiment was conducted by objective measurement and subjective assessment, and following conclusions were drawn. (1) The temperature of bed climate varied in a much narrower range compared with that of the indoor environment. (2) When operative temperature in the range of 11.3ć to 21.9ć, a thermal comfort condition could be reached on human face, with the bed system thermal resistance here of 3.73clo. (3) The temperature of mean bed climate is recommended as between 30.3ć and 32.5ć to satisfy the demand of human thermal comfort during sleep. Acknowledgements We wish to thank the National Natural Science Foundation of China (Project No. 51378411) and the Key Scientific and Technological Innovation Team of Shaanxi province, China (Grant No. 2014KCT-01) for the funding support. References [1] R.A. Trammell, A.V. Miller, Sleep and the Immune System. Encyclopedia of Sleep, 2013, pp. 568-571. [2] T. Shochat, M. Cohen-Zion, O. Tzischinsky, Functional consequences of inadequate sleep in adolescents: a systematic review. Sleep Med Rev, 18 (2014) 75-87. [3] E.H. Haskell, J.W. Palca, J.M. Walker, The effects of high and low ambient tem-peratures on human sleep stages, Electroencephalography and Clinical Neurophysiology. 51 (1981) 494-501. [4] J.S. Kum, D.G. Kim, J.I. Park, Evaluation of thermal comfort during sleeping in summer-part 4: study on indoor temperature conditions for comfort sleep, Korean J Air-Con Refrig Engineer. 19 (2007) 307-312. [5] M. Kim, C. Chun, J. Han, A Study on Bedroom Environment and Sleep Quality in Korea, Indoor. Built. Environ. 19 (2006) 123-128. [6] E.J. Van Someren, Mechanisms and functions of coupling between sleep and tem-perature rhythms, Progress in Brain Research. 153 (2006) 309-324. [7] J.W. Choi, Bed climate of Korean using ondol heating system. J. Therm. Biol. 18 (1993) 399-403. [8] R.N. Ikeda, K. Fukai, Effects of different bed sheets on bed climate and thermal response, Japan Journal of Nursing Science. 2 (2005) 51-55. [9] R.N. Ikeda, K. Fukai, K.O. Mizuno, Infant’s bed climate and bedding in the Japa-nese home, Midwifery. 28 (2012) 340–347. [10] K. Yoshihito, T. Tadahiro, I. Jin, Radiative and convective heat transfer coefficients of the human body in natural convection, Build. Environ. 43 (2008) 2142-2153. [11] Z.P. Lin, S.M. Deng, A study on the thermal comfort in sleeping environments in the subtropics—Measuring the total insulation values for the bedding systems commonly used in the subtropics, Build. Environ. 43 (2008) 905-916. [12] ASHRAE. ANSI/ASHRAE Standard 55-2010, Thermal Environmental Conditions for Human Occupancy. Atlanta: American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Inc. 2010. [13] A. Muzet, J. Ehrhart,, J.P. Libert,, V. Candas, The effect of thermal environ-ment on sleep stages. In: Indoor Climate: Effect on Human Comfort, Performance and Health, Copenhagen, Danmark, 1979, pp. 753-761. [14] F.H. Jr. Rohles, D.M. Munson, Sleep and the sleep environment temperature, Journal of Environmental Psychology. 1 (1981) 207-214.
