Sleepless Nights

Read the full report: Sleepless Nights
Download the data: .xlsx

Introduction

Global mean temperatures have risen by more than 1.3°C since 1850, and set a new record in 2023. The primary cause of this warming is rising levels of carbon dioxide in the atmosphere from burning coal, oil, and natural gas. 

Nighttime temperatures have increased even more rapidly than daytime temperatures as the world heats up. 

Hot nights make it harder for people to recover from the heat of the day, with wide-reaching and severe consequences. There is growing evidence that as nighttime temperatures rise, human sleep is being eroded around the world. A lack of quality sleep can have a range of associated impacts, including increasing the risk of many physical and mental health problems, impairing cognitive functioning, negatively affecting children’s brain development, learning and school performance, and making workplace accidents more likely.  Hot nights are also associated with increased mortality risks, with a study showing that the relative mortality risk on days with hot nights can be 50% higher than on days with non-hot nights. These climate-change-driven increases in nighttime temperatures have unequal impacts on sleep and health both within and between countries due to many factors including age, gender and access to air conditioning.

The purpose of this analysis is to analyze nighttime temperatures in the context of a warming global climate, with a focus on summer (December-February in the Southern Hemisphere, June-August in the Northern Hemisphere). To do this, we calculated the number of days where the nighttime temperature — the minimum temperature — exceeded 18°,  20°C, and 25°C. We also used the Climate Shift Index daily attribution system to quantify how climate change has influenced the number of uncomfortably hot nights. The Climate Shift Index system uses observed patterns of warming and 24 global climate models to estimate the nighttime temperature that would have occurred in a counterfactual climate without human-caused climate change. The difference between the observed number and the counterfactual number gives the number of uncomfortable nights added by climate change. For this analysis, we focus on the average over the last 10 years — 2014-2023.