Global warming is turning sleep into a global health crisis

Global warming is causing sleep to become a significant health concern worldwide. On July 13, 1995, in Chicago, temperatures soared to 41.1°C during the day and averaged 27°C at night, with a heat index reaching 52.2°C. Without the usual nighttime cooling, the body had difficulty releasing the heat accumulated throughout the day. By July 20, over 700 people had died, marking it as one of the deadliest such events in American history.

This episode highlights a less visible but crucial aspect of global warming's impact: its effects on sleep. While climate change is often associated with droughts, wildfires, water scarcity, and rising sea levels, it is also disrupting one of our most fundamental biological rhythms - sleep. The consequences of excessive night-time heat extend beyond mere discomfort, as they can impair the body's essential processes, including tissue repair, immune regulation, memory consolidation, emotional stability, and cardiovascular and metabolic health.

Sleep is not merely a passive state; it is a complex biological process that requires a specific thermal environment for optimal functioning. The body initiates a controlled decline in core temperature before sleep begins, allowing heat to move from internal organs to the skin where it can dissipate. This cooling process is crucial for initiating sleep, as it forms part of the physiological signals that guide the brain from wakefulness to slumber.

When night-time temperatures remain high, the body struggles to shed heat efficiently, particularly when humidity is also elevated. The lack of a temperature gradient between the body and its environment weakens the natural cooling process. The body's internal clock signals for sleep, but the surrounding heat hinders this vital physiological mechanism. As ambient temperatures rise beyond what the body can comfortably tolerate, both falling asleep and maintaining sleep become increasingly difficult.

The quality of sleep is more important than its duration, as it involves cycles of rapid eye movement (REM) and non-rapid eye movement (NREM) sleep. NREM sleep progresses through stages, culminating in a deep and restorative stage associated with tissue repair and immune function. REM sleep follows, associated with dreaming, memory consolidation, and information organization.

However, these sleep cycles are disrupted by hotter nights, leading to less restorative sleep and impairing the body's ability to repair itself effectively. The result is a cycle of poorer sleep quality, which in turn exacerbates the health risks associated with global warming.