The effects of increased bedroom air temperature on sleep and next-day mental performance

Size: px

Start display at page:

Download "The effects of increased bedroom air temperature on sleep and next-day mental performance"

Paul Davis
6 years ago
Views:

1 The effects of increased bedroom air temperature on sleep and next-day mental performance Peter Strøm-Tejsen 2, Sigrid Mathiasen 1, Marlene Bach 1, Steffen Petersen 1,* 1 Department of Engineering, Aarhus University, Denmark 2 Department of Architectural Technology and Construction Management, Copenhagen School of Design and Technology (KEA), Denmark * Corresponding stp@eng.au.dk SUMMARY The sleep quality and next-day performance of subjects sleeping alone in their own home were evaluated when the bedroom air temperature was changed from the personal preferred temperature. Twenty subjects, half of them women, slept for one week at their individually preferred bedroom air temperature and for one week at a temperature that was 2-3 K higher. Sleep quality was assessed using questionnaires and wristwatch-type actigraphs. Next-day mental performance was quantified using a logical reasoning test and a memory test. The results show significant negative effects of an increased bedroom temperature on subjectively assessed sleep quality and on the score from the Groningen Sleep Quality Scale. Significant negative effects were also found on assessed freshness of air, skin and lip dryness. The subjects felt significantly warmer in the room and under the duvet in the warmer condition. No significant differences were found for two objectively measured sleep quality parameters (sleep latency and sleep efficiency) or for next-day mental performance. PRACTICAL IMPLICATIONS Almost half of the Danish population suffers from poor sleep quality, and guidelines currently suggest that bedroom air temperature should be the same as for daytime activities. Discovering how sleep is affected by the bedroom temperature might be an essential step towards more effective use of energy in dwellings for 8/24 hours of each day. KEYWORDS Bedroom air temperature; Sleep quality; Performance; Field experiment 1 INTRODUCTION A normal sleep process is essential for a person s health and well-being, and there is a strong relationship between sleep and brain function the day after; logical reasoning and memory are especially vulnerable to lack of sleep. Several factors are known to interfere with the normal sleep process, but no clear effects of bedroom air temperature on sleep and next-day performance have yet been demonstrated. Shinn (1932) observed the total sleep time of 30 children aged 1-5 years over one month and found that the longest sleep occurred under conditions with moderate air temperature, and that the shortest sleep occurred during the hottest days. The optimal sleep temperature was between 21.7 and 22.8 C. Muzet et al. (1984) reported that temperatures higher and lower than the neutral air temperature decreased Slow-Wave Sleep (SWS) and Rapid Eye Movement (REM) sleep. Kim et al. (2010) used sleep apnoea as a measure of the sleep quality of 24 women of all ages during winter, spring and summer periods. The best sleep quality in

Subjective physiological data indicated that 20 C was the most comfortable temperature when awake while 23 C was the most satisfactory temperature during sleep. Lan et al.

2 these periods was reached at seasonally different average air temperatures: 24.4 C in spring, 22.7 C in winter and 28.6 C in summer. Pan et al. (2012) investigated the quality of sleep of four men and four women, all young and healthy, under different ambient temperatures (17, 20 and 23 C) in a test chamber. Subjective physiological data indicated that 20 C was the most comfortable temperature when awake while 23 C was the most satisfactory temperature during sleep. Lan et al. (2014) evaluated the sleep quality of nine females and nine males, all young and healthy, as one group sleeping in a test chamber three nights but with different ambient temperatures (23, 26, and 30 C). The subjects felt thermally neutral at 23 C when awake but reported significantly better sleep at 26 C than at 23 C and 30 C. Wang et al. (2015) studied the sleep quality of six male students and six female students, all young and healthy, sleeping in a test chamber for five nights with ambient temperatures of 10, 13, 15, 18 and 20 C, respectively. The results indicated that the thermally neutral temperature for presleep and post-sleep thermal sensation votes was 18.3 C. A bedding temperature of C and a corresponding indoor operative temperature of C during sleep were reported as comfortable. There is thus no consensus on the optimal ambient bedroom temperature but most of the evidence suggests a moderate temperature (20-26 C) and that a warmer or colder temperature can affect sleep negatively. It should be noted that the studies have limited practical applicability, primarily due to having too few subjects in each study and that experimental conditions were strictly controlled. Another limitation is that all subjects in the studies were exposed to the same temperatures, even though it is widely recognised that the conditions for thermal neutrality may differ considerably from person to person (Fanger, 1973). None of the above studies investigated how the interventions affected the next-day performance of the subjects. The work presented in this paper seeks to provide evidence on whether the sleep quality and next-day performance of subjects sleeping alone in their own homes are affected when the bedroom air temperature was increased above their individually preferred temperature. 2 METHOD Experimental settings and subjects Experimental data was obtained in a series of field intervention experiments in which 10 female and 10 male students aged years participated as subjects. All subjects lived in similar single-occupancy dormitory rooms located in Aarhus, Denmark (Figure 1). Eight rooms had windows in a façade facing south-west while 12 rooms faced north-east. The subjects had lived in their rooms for a period of between 1.5 months and 5 years. The subjects were physical healthy, none had doctor-diagnosed sleep disorders, and all except one were ethnic Danish (the non-danish student was Austrian). Six subjects had pollen/dust allergy, four were smokers, and two had been diagnosed with a mild psychiatric illness. Two females and four males were overweight, and one male was obese according to the Body Mass Index. a) b) c) Figure 1. Dormitory used for the experiment. a) Building, b) Façade, c) Room layout.

3 Intervention The experiment was carried out as an intervention on the preferred bedroom air temperature of each individual subject. The subjects were exposed to their preferred air temperature (normal), a colder (2-3 K below normal), or a warmer air temperature (2-3 K above normal) in balanced order (see Table 1). Results from the colder condition are not reported in this paper. Each subject was exposed 3 times, once to each condition, each time for one week. Every Friday a text message was sent to the subjects with instructions on how to adjust their district heating radiators to meet the conditions in Table 1. After one or two days the subjects used a simple thermometer to measure and report the new actual air temperature in their room to the researchers. In a few cases the subjects were asked to adjust the radiator further to reach the desired air temperature. The three first nights after the adjustment (Friday to Monday) were considered as an adaptation period for the new condition, and experimental data were then collected from four nights (Monday to Friday). The interventions were carried out during two periods of three weeks (February 27 th to March 20 th and April 10 th to May 1 st, 2015). The reason for having two different periods was shortage of measurement equipment. The subjects were asked to maintain their habitual life style during the experiment. However, they were asked to minimize their intake of caffeine and alcohol as much as possible or at least to keep the intake uniform. They were also asked to sleep with their window closed. Table 1. Intervention schedule and temperature. Period No. of subjects Week 1 Week 2 Week 3 4 Normal Warm Cold* March 3 Cold* Normal Warm 3 Warm Cold* Normal 4 Normal Warm Cold* April 4 Cold* Normal Warm 2 Warm Cold* Normal *The results from the colder condition are not reported in this paper. Physical measurements The room air temperature and relative humidity were logged every five minutes using a HOBO Data Logger U (temperature ±0.5 K, RH ± 2.5%). The CO 2 concentration was measured using a silicon-based single-beam dual-wavelength sensor (Vaisala GM 20, ±45 ppm + 2% of the reading) and was also logged every five minutes by the HOBO logger. The instruments were placed close to the bed and at a suitable distance from the window, door and any heat generating equipment (computers and alike). Physical activity during sleep was recorded as movement data recorded on wristwatch-type actigraphs worn by the subjects on their non-dominant wrist while sleeping (Actigraph GT3X+, Sensitivity: 3 mg/lsb, estimated error for sleep efficiency: 1%). Sleep latency (time to fall asleep) and sleep efficiency (proportion of time in bed spent asleep) were then estimated from these data, using commercially-available software (Pollak et al. 2001). Questionnaires The subjects completed an online questionnaire between 20 and 30 minutes after waking up on the mornings between Monday and Friday. The questionnaire contained the 15 true/false questions from the Groningen Sleep Quality (GSQ) Scale developed to evaluate sleep quality during the previous night (Meijman et al. 1988). The questionnaire also contained questions where the subjects could assess their sleep quality and general well-being in the morning on continuous visual analogue scales, various questions related to sleep latency, visual analogue scales for subjective assessment of the indoor climate and sick building syndrome symptoms,

and questions about their sleeping garments, and their alcohol and caffeine intake the evening before. Performance tests Two online performance tests were included in the morning questionnaire.

The subjects had to press false or true to statements that were either active or passive sentences, see Figure 2a.

4 and questions about their sleeping garments, and their alcohol and caffeine intake the evening before. Performance tests Two online performance tests were included in the morning questionnaire. The first was a one-minute logical thinking test in the form of grammatical reasoning (Baddeley, 1968). The subjects had to press false or true to statements that were either active or passive sentences, see Figure 2a. Performance was measured in terms of the total score, the number of trials and the highest number of consecutive correct answers. The second test was a one minute and 20 seconds memory test inspired by the Monkey Ladder (Cambridge Brain Sciences, n.d.) where boxes with running integers starting from one appear on the screen. The integers in the boxes then disappear after 1.5 seconds times the number of boxes. The task of the subject is now to click on the empty boxes in the correct numerical order. If the subject answers correctly, the level of difficulty is increased for the next question. If the subject answers wrongly, the level of difficulty is decreased. The order of integers and placement of boxes were random, see Figure 2b. The subjects scored +1 point for a correct answer and lost a point for each incorrect answer. Performance was measured as the total score, the number of trials, and the highest number of consecutive correct answers. The subjects were instructed to practice both tests prior to the experiment to avoid a learning effect that might bias the results. a) b) Figure 2. Examples from the performance tests. a) Logical thinking, b) Memory test. Data processing The measurements of temperature, relative humidity and CO 2 concentration were assumed to be normally distributed and are presented in the paper as average values. The data from the actigraphs, the questionnaire and the performance tests were tested for normality using the Ryan-Joiner s Test (P>0.10). Data from the performance tests were normally distributed and a paired t-test was used to analyse the within-subject difference between the two experimental conditions. Data from the questionnaire and the actigraphs were not normally distributed and the nonparametric Wilcoxon Matched-Pair Signed-Ranks Test was therefore used. The P- values reported in the Results section are for a two-tailed test of the difference between conditions in the 4-day mean values. The accepted level of confidence in statistical tests conducted was P< RESULTS AND DISCUSSION Physical measurements of the indoor environment Table 2 shows the mean air temperature, relative humidity and CO 2 concentration obtained in the two conditions. The desired room air temperature of 2-3 K warmer than the normal temperature was not obtained in all rooms. The mean temperature for the normal condition was 22.0 C and the temperature was on average 2.0 K higher in the warm condition. The

5 mean relative humidity (RH) was slightly lower in the warm condition as a natural result of the increased temperature. The mean CO 2 concentration often varied between conditions but was of the same magnitude across the conditions. Table 2. Mean values from the physical measurements for each condition. Period Subject Mean air temperature [ C] Relative humidity [%] CO 2 concentration [ppm] Normal Warm Normal Warm Normal Warm March April Mean Morning questionnaire and actigraph data In terms of alcohol and caffeine consumption in the six hours before bedtime, seven subjects reported that they did not ever consume either, nine subjects reported consumption one or two times, and the remaining four subjects reported consumption three-seven times out of the possible eight evenings. None of the subjects chose to adjust their sleeping garments or their duvet during the experiment. Table 3 shows the results of the statistical analysis. The subjects had a general tendency to sleep better in their normal condition than in the warmer condition. In the normal condition compared to the warm condition all subjects reported a significantly better score on the Groningen Sleep Quality Scale (P<0.0298), better subjectively assessed sleep quality (P<0.0019), and there was a tendency to feel more rested in the morning (P<0.0894), see Figures 3-5. These differences were significant for male subjects considered separately, but not for female subjects. Although not statistically significant (P<0.0559), there was a tendency for better well-being in the normal condition. This tendency was only for the male subjects when each gender was considered separately (P<0.0506). The outcome of the experiment supports earlier findings that a midrange air temperature (normal condition) results in the best sleep (Shinn, 1932; Muzet et al. 1984; Kim et al. 2010; Pan et al. 2012; Lan et al. 2014; Wang et al. 2015). Preferred bedroom temperature varies widely as a function of sleepwear, bedcover insulation and drape, and mattress insulation. The

6 mean bedroom air temperature in the normal condition during the experiment reported in this paper was C. Table 3. Results from the statistical analysis of data from the morning questionnaire and the actiwatches with P-values from the Wilcoxon Matched-Pair Signed-Rank Test. Variable All Comments Females Males Sleep Groningen Sleep Quality Scale Better in the normal condition Sleep quality (questionnaire) Better in the normal condition Sleep efficiency (actigraph) Sleep latency Subjective Objective (actigraph) SBS symptoms Headache Throat dryness More dry in the warm condition Blocked nose Nasal dryness More dry in the warm condition Mouth dryness More dry in the warm condition Skin dryness More dry in the warm condition Lip dryness More dry in the warm condition Sleep environment Room temperature Warmer in the warm condition Temperature under duvet Warmer in the warm condition Freshness of air More fresh in the normal condition Air humidity Air movement Noise Others General well-being Better in the normal condition Rested in the morning More rested in the normal condition Freshness yesterday Ability to concentrate yesterday P-values are 2-tailed. Bold: P<0.05 GSQ score Normal P< Warm Disturbed sleep Figure 3: Score from the GSQ scale. Sleep Quality Normal sleep 0 Normal P< Warm Very good Figure 4: Ratings of sleep quality. Rested Very bad 0 Normal P< Warm Figure 5: Ratings of being rested. Well rested Tired out The statistical analysis of the actigraph data used for objective measures of sleep efficiency and latency did not yield any significant differences between conditions. Laverge et al. (2012) did not obtain statistically significant results from actigraph data when they studied the effect of ventilation on sleep, although in a similar field study on ventilation and sleep by

7 Strøm-Tejsen et al. (2016), significant results were found. Actigraphy makes it possible to evaluate the sleep quality of subjects in their normal sleep environment, but in future field studies of the influence of the sleep environment on sleep a more sensitive type of actigraph should perhaps be used. Significant negative effects of the warm condition compared to the normal condition were found for subjectively assessed skin dryness (P<0.0429) and lip dryness (P<0.0313). Throat (P<0.1034), nasal (P<0.1221) and mouth dryness (P<0.0872) tended to be more marked in the warm condition. These symptoms of dryness can be due to the lower relative humidity in the warmer condition (Wyon et al. 2006). Field investigations by Sundell and Lindvall (1993) concluded that indoor air humidity might not be an important factor for the sensation of dryness, since the main factor is the level of pollutants in the air. This is not the case in the present investigation, since the ventilation rate and therefore the level of pollutants in the air was the same for the normal and the warm condition. The perceived freshness of air was better in the normal condition for all subjects (P<0.0383). This difference was significant only for the female subjects when each gender was considered separately (P<0.0415). The direction of the effect was expected, due to the lower enthalpy in the normal condition (Fang et al. 2004). Significant differences between conditions in the expected direction were also found for the subjective assessment of the room air temperature (P<0.0001) and of the temperature under the duvet (P<0.0001). No differences between conditions were found for air humidity, air movement or noise. No statistically significant differences between the two conditions were found for the tests of mental performance, see Table 4. However, the subjects generally obtained lower scores during their first experimental week, which indicates that they did not practice the test prior to the experiment as they were instructed to do. This learning effect will have reduced the sensitivity of the comparison between conditions. Table 4. Results from the tests of logical reasoning (1) and memory (2). Variable All Test 1 Total points Test 1 Maximum trials Test 1 Correct in a row Test 2 Total points Test 2 Maximum trials Test 2 Correct in a row P-values are from paired t-test, 2-tailed. Experimental design The study was performed in the subjects own homes, in their normal sleeping environment with their preferred sleepwear and bedcover, which added to the realism of the study, but it was not possible to fully control the physical parameters of the sleep environment and the subjects were not completely blinded to the intervention. The number of confounding variables such as outdoor noise level, and indoor and outdoor pollution sources was reduced by using identical student dormitory rooms. There was no disturbance from other people in the room since the rooms are designed for single occupancy. However, the probability of obtaining significant results was reduced by individual daily variation due to the student lifestyle, with no regular schedule during the week. 4 CONCLUSIONS The results of this field experiment show that assessed sleep quality and the score from the Groningen Sleep Quality Scale for subjects sleeping alone in their own homes were negatively affected when the air temperature was increased to 2 K above each subjects

8 normally preferred bedroom temperature. Symptoms of dryness were more marked in the condition with increased temperature. There was a tendency for the subjects to feel more rested and to experience better well-being the day after sleeping at their normal preferred bedroom temperature. No statistically significant differences between the two conditions were found using tests of next-day mental performance. ACKNOWLEDGEMENT The authors express their gratitude to the Technical University of Denmark for making available the equipment for physical measurements, to the National Research Centre for the Working Environment and Aarhus University Sport Science department for making the actigraphs available, and to David Wyon for help regarding the statistical analysis. REFERENCES Baddeley A A 3-min reasoning test based on grammatical transformation. Psychonomic Science, 10(10), Cambridge Brain Sciences, n.d. Monkey ladder. Available at: (Accessed Feb. 2016). Fang L, Wyon D.P, Clausen G, and Fanger P.O Impact of indoor air temperatures and humidity in an office on perceived air quality, SBS symptoms and performance. Indoor Air, 14(Suppl. 7), Fanger P.O Assessment of man's thermal comfort in practice. British Journal of Industrial Medicine, 30, Kim M, Chun C, and Han J A study on bedroom environment and sleep quality in Korea. Indoor and Built Environment, 19(1), Lan L, Pan L, Lian Z, Huang H, and Lin Y Experimental study on thermal comfort of sleeping people at different air temperatures. Building and Environment, 73, Laverge J, Novoselac A, Corsi R, and Janssens A Experimental assessment of ventilation in the bedroom: physiological response to ventilation and impact of position on rebreathing. In: Proceedings of the 5th IBPC, Kyoto, Japan. Meijman T.F, de Vries-Griever A.H, and de Vries G.G The evaluation of the Groningen Sleep Quality Scale. Heymans Bulletin, HB EX. Muzet A, Libert J, and Candas V Ambient temperature and human sleep. Experientia, 40(5), Pan L, Lian Z, and Lan L Investigation of sleep quality under different temperatures based on subjective and physiological measurements. HVAC&R Research, 18(5), Pollak C.P, Tryon W.W, Nagaraja H, and Dzwonczyk R How Accurately Does Wrist Actigraphy Identify the States of Sleep and Wakefulness? Sleep, 24(8), Shinn A A study of sleep habits of two groups of preschool children, one in Hawaii and one in mainland. Child Development, 3(2), Strøm-Tejsen P, Zukowska D, Wargocki P, and Wyon D.P The effects of bedroom air quality on sleep and next-day performance. Indoor Air, doi: /ina Sundell J. and Lindvall T Indoor air humidity and sensation of dryness as risk indicators of SBS. Indoor Air, 9(3), Wang Y, Yanfeng L, Cong S, and Liu J Appropriate indoor operative temperature and bedding micro climatetemperature that satisfies the requirements of sleep thermal comfort. Building and Environment, 92, Wyon D.P, Fang L, Lagercrantz L, Fanger P.O Experimental determination of the limiting criteria for human exposure to low winter humidity indoors. (RP-1160). HVAC&R Research, 12(2),

Keywords: Indoor air quality, Sleep quality, Actigraph, Ventilation rate

Keywords: Indoor air quality, Sleep quality, Actigraph, Ventilation rate Topic A8: IAQ & perceived air quality THE EFFECT OF AIR QUALITY ON SLEEP Peter Strøm-Tejsen 1,*, Pawel Wargocki 1, David P. Wyon 1, and Anna Kondracka 1,2 1 International Centre for Indoor Environment