Ambient warming and human sleep duration and timing: a worldwide case-crossover study using big data from wearable devices

Abstract

BACKGROUND AND AIM: Only limited self-report evidence links outdoor temperatures with alterations in sleep. We aimed to assess the attributable impact of nighttime minimum temperature on objectively recorded sleep outcomes. METHODS: In a global natural experiment, we linked over 10 billion minute-level sleep measurements comprising 7.41 million nightly sleep records (n=47,628) from accelerometer-based wristbands to daily meteorological data across 68 countries between 2015 and 2017. We used person-level longitudinal panel fixed effects models to isolate the within-person sleep impact attributable to nighttime temperature. The primary endpoints were sleep timing and total time asleep. RESULTS:Controlling for potential device-specific, person-specific, location-specific, and temporal confounds, warmer temperatures were associated with reduced sleep duration across the observed temperature distribution. Within-person sleep duration declined by 6.11 minutes per every 10°C increase when nighttime minimum temperatures exceeded 10°C. During nights over 25°C, sleep onset was delayed by 5.92 minutes, sleep offset advanced by 2.51 minutes and the probability of short 7hr sleep increased by 3.50 percentage points compared to the median temperature reference range of 5-10°C. The negative relationship between a 1°C increase in ambient temperature and sleep duration was more acute for the elderly compared to middle aged adults, for females compared to males, and residents of lower income countries compared to residents of high income countries. There was no evidence for short term acclimatization or intraday sleep substitution with daytime rest. CONCLUSIONS:Warmer nighttime ambient temperatures were associated with reduced and delayed sleep. This evidence suggests a plausibly causal pathway through which rising temperatures may contribute to global health inequalities if unmitigated climate change continues. Employing 21 global climate models, we project that climate change may cumulatively erode adult sleep by 23.40 hours (warmest regions) to 5.40 hours (coldest regions) annually under a high emissions scenario, barring further adaptation. KEYWORDS: Climate, Big Data, Mental health outcomes, Causal inference, Sleep