Table 1 Summary of studies

Huang, 2022 [24]

Xinyi, Xuzhou, China

Randomized controlled trials

n = 41; 60% female, mean age = 58.8 years

Participants were split into three groups of 10 and one of 11, one control, and three experimental. Baseline health metrics were taken on a normal-temperature day. For 5 days during a subsequent heat wave, three groups each received one of the following interventions: education about health during heat waves, subsidies for electricity costs of running an AC or fan, or daytime water spraying of homes in an attempt to cool interior temperatures

Use of an unspecified smart band that measured TSD, DSD, and LSD. The paper did not specify how the smart band measured these durations

None

A wrist blood pressure monitor measured DBP and SBP and HR of participants. Both metrics were measured three times every morning for the 5-day study period

In the control group, DBP and SBP elevated from baseline during the heat wave, with SBP increasing significantly on days 1 and 2 by 5.33 mmHg (95% CI: 3.38–7.30; P = 0.01) and by 4.92 mmHg (95% CI: 2.74–7.09; P = 0.02), respectively

HR elevated from baseline and on day 1 and lowered to near baseline by day 5. DSD decreased significantly in the first 3 days by − 0.48 h (95% CI: − 0.61, − 0.34; P = 0.00), − 0.36 h (95% CI: − 0.51, − 0.21; P = 0.01), and − 0.25 h (95% CI: − 0.37, − 0.12; P = 0.05), respectively

In the cooling-spray group, SBP increased significantly on day 1 by 3.18 mmHg (95% CI: 1.73, 4.63; P = 0.03) and on day 2 by 3.34 (95% CI: 1.76, 4.93; P = 0.04) before gradually declining and returning to baseline. DSD was reduced significantly on day 2 by − 0.21 h (95% CI: − 0.31, 0.11; P = 0.05)

The experimental group interventions were not fully described. Information on occupation and AC use was not collected in an initial questionnaire, nor was information on ability to pay for AC, which would shed light on the efficacy of subsidies

Kim, 2020 [25]

Rural areas in southern South Korea: Gijang, Busan; Imsil, Jeollabuk-do; Gwangyang, Jeollanam-do; and Namhae, Gyeongsangnam-do

Cohort study

n = 104; 72.1% female, mean age = 79.6 years

All participants were exposed to the 2018 heat wave in South Korea (August 1–19). Indoor temperature and relative humidity were measured twice a day (morning and afternoon) for 3 days. Outdoor temperature and relative humidity were retrieved from the Korean Meteorological Administration website for each study area, with average values between 9 AM through 12 PM used as morning data and 1 PM through 5 PM used as afternoon data

None

Number of hours of sleep during the prior night was self-reported to investigators on days in which temperature, relative humidity, and health measures were taken

Body temperature measured by infrared thermometer; DBP and SBP measured twice (morning and afternoon) per day using a sphygmomanometer

DBP decreased significantly (P < 0.001) in subjects with hypertension, with a 1 °C increase in indoor temperature decreasing DBP by 0.44 mmHg (95% CI: 0.04–0.84 mmHg). The association between indoor temperature and SBP was positive but not significant. Number of hours of sleep decreased with indoor temperature by 0.036 h (95% CI: − 0.138, 0.067 h); however, this result did not reach statistical significance

No analysis was performed to determine if subjects who reported fewer hours slept had significant differences in BT, DPB, or SBP

Yan, 2022 [26]

Shanghai, China, in controlled hospital bedroom setting

Controlled, crossover trial

n = 16, 50% female, mean age 72 years

Each participant was assigned to one of four experimental conditions established in a 2 × 2 experimental design: hospital-based bedrooms were heated to either 27° or 30° C and used or did not use a mechanical ventilation system to provide the room with filtered outdoor air. Participants spent five nonconsecutive nights (one adaptive and four observed) sleeping in their assigned room, with a 3-day interval between experimental nights

Total sleep time, sleep efficacy, sleep onset latency, time awake and duration of sleep stage, measured using EEG, bilateral EOG, and chin EMG

Questionnaire each morning measuring calmness of sleep, ease of falling asleep, ease of awakening, freshness after awakening, and sleep satisfaction on 5-point scales

Heart rate and heart rate variability measured by ECG; DBP and SBP measured before and after sleep using a sphygmomanometer

Compared to 27 °C, individuals at 30 °C had a significantly increased time awake (MD = 15.9 min in MV setting, MD = 38.1 min in NMV setting, P = 0.01), less total sleep time (MD = 14.5 min in MV setting, MD = 38.1 min in NMV setting, P = 0.01), less sleep efficiency (MD = 3% in MV, 8% in NMV, P = 0.01), and less REM sleep (MD = 7.4 min in MV setting, MD = 3.3 min in NMV setting, P = 0.05); heart rate variable significantly differed (MD = 0.7 bpm in MV setting; 0.7 bpm in NMV setting, P = 0.02), and DBP (P = 0.01) and SBP (P < 0.001) significantly decreased with increases in duration of deep sleep defined by REM

Rigorously designed experimental study