The quantity and quality of sleep affects human health1–4 and cognitive performance5–7. The thermal environment, characterized by the dry-bulb air temperature, mean radiant temperature, relative humidity, and air speed, affects sleep quality8,9. The strong link between sleep and thermoregulation means both excessively hot and cold thermal conditions have a negative impact on sleep outcomes10. Feeling too warm during sleep increases wakefulness and decreases time in shortwave sleep prominent in the initial sleep segments, and rapid eye movement (REM) sleep in the later stages11. Feeling too cold during sleep primarily effects the later stages of sleep, where REM is dominant10 and significantly affects heart rate variability during sleep12 which may contribute to higher frequencies of myocardial infarctions (i.e., heart attacks) in the morning13 and in winter14. The negative effect can be compensated with bedding insulation, for example, a study report no significant reduction in sleep quality in ambient temperatures as low as 3°C due to bed covers maintaining a near constant bed thermal environment12.
Despite the importance of the thermal environment to sleep, there are limited building regulations or guidelines that specifically address the design temperature in sleeping environments. Therefore, design practitioners generally assume that the same conditions for thermal comfort during waking hours apply to sleep. In the United Kingdom, the Chartered Institution of Building Services Engineers’ (CIBSE) TM59 Design Methodology for the Assessment of Overheating Risk in Homes recommends that the operative temperature in the bedroom from 10 p.m. to 7 a.m. shall not exceed 26°C for more than 1% of annual hours15. Nicol, who was involved in the standard’s development, later suggests that this criteria may be highly conservative as people sleep comfortably at temperatures of 29-31°C within their personal bed space16. Lomas et al.17 also finds that the TM59 criterion suggests a much higher prevalence of overheating than was reported by the English Housing Surveys (EHS).
Public health agencies have a role in communicating evidence-based guidelines on coping with extreme temperatures. Currently, most public health guidance focuses on preventing adverse health effects from overheating18–21. With regards to the home and personal adaptations, these guidelines are often limited to reducing use of internal heat gains like ovens and lights, opening windows at night (if safe), using air-conditioning, staying hydrated, and dressing in light clothing. Fewer agencies provide guidelines on preventing adverse health effects from a winter power outage or extreme cold22,23. Guidance with regards to the home and personal adaptations focuses on using gas-powered generators with caution and dressing in warm layers. In both cases, there are no specific guidelines on sleeping in extreme temperatures. As a result, popular consumer publications attempt to fill this communication gap, demonstrating public interest in the topic, but their guidelines are often based on reader anecdotes and not on sound scientific research.
Conventional heating, ventilation, and air conditioning (HVAC) systems are a common and effective way to regulate the thermal conditions inside residential buildings. For example, residential air conditioning is the main reason for the reduction of 75% mortality in the US due to excessive heat24. HVAC technologies have high penetration in the United States, where over 95% of homes have some form of space heating and 88% have some form of space cooling25. However, space heating and cooling is energy intensive (power rating ~3 kW for cooling and ~20 kW for heating) and historically represents over 50% of energy end use in residential buildings26. In a survey of New York City residents, 91% of respondents with air conditioning at home had it installed in their bedroom27. Bedroom air conditioning is especially energy intensive because it typically operates continuously throughout the night. Survey data has recorded this behavior in New York City27, Hong Kong28, China29, and Singapore30. This indicates a priority for comfortable sleeping environments in many different contexts. Yet HVAC systems are cost-prohibitive for many households. Over a quarter of the 123.5 million households in the U.S. report energy insecurity, which may result in leaving the home at uncomfortable temperatures (12.2 million households), receiving a disconnect or delivery stop notice (12.4 million households), or unable to use heating (5.1 million households) or air-conditioning equipment (6.4 million households)25. When taking a broader world view, access to HVAC systems as well as reliable and affordable energy is limited to only a part of the world population. According to the International Energy Agency (IEA)’s report on the future of cooling, of the 2.8 billion people living in the hottest parts of the world, only 8% currently have access to air conditioning31.
Climate change further challenges the viability of conventional HVAC systems as a means towards comfortable and healthy sleep in several ways. First, diurnal warming asymmetry32 means nighttime temperatures are warming faster than daytime temperatures in much of the world, and this could be a problem in climates dominated by cooling requirements. Atypically warm nighttime temperatures are associated with elevated mortality33 and poor sleep quality34 particularly among those with limited ability to cope, such as the low-income and elderly persons. Higher nighttime temperatures will increase the energy consumption of existing air conditioners and drive installation of new air conditioners, further exacerbating climate change and urban overheating35; it will also reduce heating needs. Second, the greater frequency and intensity of climate change impacts like heat waves, cold snaps, and wildfires increases the probability of power disruptions. These events compound the disaster36 as was seen in British Columbia, Canada and the U.S. Pacific Northwest in summer 2021 and in Texas in Winter 202137. The lack of resiliency and energy intensity of conventional HVAC systems necessitate an alternate strategy, such as personal comfort systems (PCS) to maintain comfortable and healthy indoor air temperatures during sleep.
PCS are thermal systems that heat and/or cool the individual rather than the entire space and are under the individual’s control38. Most research on applications of PCS focus on increasing thermal comfort and reducing energy consumption in office buildings39. However, PCS may be well-suited for sleeping due to the stationary nature of the person. They are cheaper to operate as they require less power (1-100 W) than conventional HVAC systems. Some devices are so efficient that they can be battery operated, making them resilient to utility power interruptions. PCS can also be implemented as part of a strategy to reduce building HVAC energy consumption by extending air temperature set points40–42.
A few studies have reviewed the impact of PCS on sleep quality and thermal comfort. Lan et al43 found localized cooling of the back and/or head with a hypothermia blanket significantly improved objective and subjective measures of sleep quality in a relatively hot environment (32°C). Other studies found that head cooling by means of a pillow with a chemical cooling medium improved sleep quality44 and decreased the sweat rate45, a physiological measure of thermal strain. Increased air movement with fans in a relatively hot environment (30°C) maintained thermal comfort and sleep quality compared to conventional air-conditioning set to 27°C46. In cold ambient temperatures (5°C), Song et al.47 found a partial-body heating system with a heated electric blanket improved thermal comfort and sleep quality. Okamoto-Mizuno et al.48 also found a heated electric blanket to decrease cold stress in a 3°C environment during sleep.
Fans are a relatively common amenity in U.S. homes, with over 70% of households having at least one ceiling fan and over 40% have at least one floor or window fan25. Elevated air movement using fans can replace or augment cooling from air conditioning in normal and extreme thermal conditions. The elevated air movement from fans increases thermal comfort at higher air temperatures by accelerating convective and evaporative heat loss from occupants. Other benefits of fans include improved air distribution, improved perceived air quality, HVAC first cost savings, and energy savings49–51.
Although the power consumption of PCS is significantly less than conventional HVAC systems, both are considered active strategies as they require an energy input. Alternatively, there are passive adaptations to improve sleep quality that do not require energy input. Examples include change of bedclothes and bed type, and changing posture16. In a hot environment, a rope bed, such as the charpai in South Asia, may provide more cooling than a conventional mattress. Other behavioral adaptations include migrating to different levels of a building based on the principle of heat rising i.e., sleeping downstairs or on the floor in the summer and sleeping upstairs in the winter, or even sleeping outside to take advantage of radiative sky cooling.
We know that sleep is crucial for human health and wellbeing, and that the thermal environment can affect sleep quality. The current approach of relying on conventional HVAC to control the thermal environment is challenging from an energy, sustainability, and affordability perspective – issues further exacerbated by climate change. We do not know the role that localized interventions like PCS and other personal adaptations can play in improving sleep quality.
Approach
We used a dry heat loss thermal manikin52 in a controlled environmental chamber to evaluate the performance of 11 passive and low-energy strategies in the context of sleeping. These strategies ranged from simple measures such as changing clothing and posture, using a fan, to more advanced products like a hydro-powered mattress pad. We tested heating strategies at an indoor air temperature of 16°C and cooling strategies at 28°C. For each strategy, we measured the thermal manikin skin temperature and heat loss at 16 body segments. We then calculated the equivalent temperature, defined as the temperature in which a thermal manikin with realistic skin temperature would lose heat at the same rate as it would in the actual environment, for each of the 16 body parts52. The whole-body equivalent temperature is the area-weighted average over all body segments. The difference in equivalent temperature between the thermal manikin with heating or cooling intervention and a baseline condition gives the heating and cooling effect68. This metric allows us to quantitatively compare the different heating and cooling strategies.
To better contextualize our laboratory findings, we applied our results to two historical case studies: the 2015 Pakistan heat wave and the 2021 Texas power crisis during a cold snap. These represent extreme heat and extreme cold events where conventional HVAC systems were not available either due to lack of access or a multi-day power outage. For both case studies, we modeled both the indoor air temperature of a typical residence and the relationship between our experimentally measured heating and cooling effect and indoor air temperature. We calculated the intensity and duration of hazardous heat or cold exposure during sleep with and without top-performing heating and cooling interventions. During the case study period, we calculated hazardous heat or cold exposure as the difference between the modeled indoor temperature and WHO’s guidelines for indoor minimal risk temperature for adverse health effects53 as appropriate for the heating or cooling condition and local climate. We only considered hazardous heat or cold exposure during sleeping hours, defined as 10 p.m. – 7 a.m. per CIBSE TM59. We assume that outside of sleeping hours, individuals would have access to a different set of heating or cooling interventions.