Pilot study on the application of bedside personalized ventilation to sleeping people
Introduction
People spend about one third of their lives sleeping. Sleep is essential to help the body recover from both physical and psychological fatigue suffered throughout the day and helps restore energy to maintain bodily functions [1], [2]. The indoor environmental parameters including air temperature, relative humidity and indoor air quality (IAQ) etc. could significantly affect sleep quality. Air temperatures that are higher or lower than the neutral temperature decrease slow wave sleep (SWS) and rapid eye movement (REM) sleep, and increase the frequency and duration of wakefulness [3], [4]. Moreover, our study found that sleeping people had different thermal comfort requirements from waking people [5]. Combined with high humidity, humid heat exposure during night sleep increases wakefulness and decreases SWS, REM and melatonin secretion, probably through thermoregulatory mechanisms [6], [7]. Elevated air flow has been found to be able to reduce the heat load and facilitate sleep in a warm humid condition [8]. Although there was no clear evidence to substantiate that sleeping duration decreased with increasing levels of CO2, the findings did suggest that high levels of CO2 may hinder the duration of sleep [9]. For awaking people, studies have found that poor air quality or increased CO2 concentration causes sick building syndrome (SBS) symptoms such as increased prevalence of headache, decreased ability to think clearly, etc. [10], [11].
Many problems were found regarding the current situation of IAQ and thermal environment in bedrooms in the field surveys. The study in Korea shows that people were exposed to too low or high air temperatures and high CO2 concentration etc., when asleep [12]. Similar results were found in high-rise residences in Hong Kong that most people felt stuffy because of poor IAQ, and that approximately 60% of the respondents had experience of waking up during sleep because they felt either cold or warm even if air-conditioners were turned on in their bedrooms [13]. Measurement in the bedrooms of 500 Danish children show that 23% of the rooms experienced at least a 20-min period during the night when the CO2 concentration was above 2000 ppm and 6% of the rooms experienced concentrations above 3000 ppm [14]. Sehar found that in Singapore the overnight build-up of CO2 level in a bedroom served by a split system air-conditioning unit can be as high as 2900 ppm, due to lack of ventilation [15]. At present most Chinese homes with forced air conditioning have no mechanical supply of outdoor air.
Personalized ventilation (PV) systems may be served as a method to improve thermal comfort and IAQ in bedrooms, due to their relatively good performance in local thermal environment and air quality control [16], [17], [18]. The current investigations or applications of PV are mainly for awaking people, but not for sleeping people, although the immobility of the latter and the relatively small occupied space are the favorable conditions for the use of PV. Pan et al. probably are the pioneers who reported the energy saving performance of a novel bed-based task/ambient conditioning (TAC) system that supplied air with two symmetrically placed plenums on both sides of a mattress bed; their study demonstrated that the use of the bed-based TAC system could achieve energy saving, compared with the use of a full volume air conditioning system [19]. However, to the best of our knowledge, there is no report covering the effects of TAC or PV on sleeping people.
Unlike the bed-based TAC that ventilates the whole body, we proposed to use a bedside PV system (Fig. 1) that directly supplies fresh and cool/warm air to the head and face of sleeping people. Studies on awaking people found that cooling the head is more effective in reducing thermal stress than cooling any other part of the body [20], [21]. Desruelle and Candas demonstrate that, combining with cool air breathing, face skin cooling effectively caused a reduction in heat strain [22]. The study of Okamoto-Mizuno et al. reveals that the use of cooling pillow helped to decrease the whole-body sweat rate during sleep under humid heat conditions [23]. The Krauchi et al.'s report is another important reason why we preferred the bedside PV system; their study indicates that foot skin temperature is important for good sleep, and it has to be kept comparatively higher to promote the rapid onset of sleep [24]. However, great caution should be paid when applying this bedside PV to human subjects, because the continuous ventilation time could be very long (up to 7 h or more throughout the sleeping period) and most sleeping people are defenseless with respect to their immediate environment. In this study we carried out an experiment with human subjects cautiously at thermal neural environments, mainly aiming to ascertain whether the bedside PV would cause any negative consequences to the sleeping people if it was used for a whole night time.
Section snippets
Bedside PV for sleeping people
Fig. 1 shows the sketch map of this bedside PV system. The air terminal device, i.e. the cylindroid movable panel (CMP) will supply clean air directly to the breathing zone of the sleeping people meanwhile cool/warm the face skin, under the individual control. The CMP is mounted on a movable arm-duct, which allows the position of the CMP to the sleeping people and the direction of supplying airflow to be changed by the individuals. Each PV also offers occupants the possibility to adjust the
Results
Table 1 shows the physical parameters describing the indoor environment in the bedrooms as well as the local environment near the subject's face (if the PV was turned on). The background temperature and humidity in the bedroom were similar under both conditions. When the PV was turned on, a low velocity of 0.15 m/s was kept and the temperature near the subject's face was 1 °C higher than that of the background environment; the local temperature and velocity did not deviate from the intended
Discussion
Poor sleep quality impairs cognitive performance in older adults [34], and more significantly, impacts brain function related to reward processing, risk-taking, and cognition in adolescents [35], [36]. Disturbed nocturnal sleep has also been found to be related to various adverse health problems, increasing the risk of death from cardiovascular disease [37]. Therefore, providing a good sleeping environment is important for sleep maintenance as well as daytime activities and health status.
Conclusions
A bedside PV system that supplies fresh air to the breathing zone of sleeping people was applied to human subjects for a whole night sleep. The subjects perceived to be cooler and their skin temperature decreased when the bedside PV was turned on, even if the supplying air temperature of PV (about 24 °C) was slightly higher than that of the background environment (about 23 °C). Compared with no PV, the VLF power and the LF/HF ratio throughout the sleep significantly decreased when the PV was
Acknowledgments
This work was supported by the National Natural Science Foundation of China (No. 51108260 and 51238005). The authors also would like to thank the experts and the subjects who actively participated in this study.
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