Heatwaves and human sleep: Stress response versus adaptation.

The World Meteorological Organization considers a heatwave as "a period of statistically unusual hot weather persisting for a number of days and nights". Accompanying the ongoing global climate change, sharp heatwave bouts occur worldwide, growing in frequency and intensity, and beginning earlier in the season. Heatwaves exacerbate the risk of heat-related illnesses, hence human morbidity and mortality, particularly in vulnerable elderly and children. Heat-related illnesses present a continuum from normothermic (prickly heat, heat edema, heat cramps, heat tetany) to hyperthermic syndromes (from heat syncope and heat exhaustion to lethal heat stroke). Heat stroke may occur through passive heating and/or exertional exercise. "Normal sleep", such as observed in temperate conditions, is altered during heatwaves. Brisk excessive heat bouts shorten and fragment human sleep. Particularly, deep N3 sleep (formerly slow-wave sleep) and REM sleep are depleted, such as in other stressful situations. The resultant sleep loss is deleterious to cognitive performance, emotional brain function, behavior, and susceptibility to chronic health conditions and infectious diseases. Our group has previously demonstrated that sleep constitutes an adaptive mechanism during climatic heat acclimatization. In parallel, artificial heat acclimation procedures have been proposed in sports and military activities, and for the elderly. Other preventive actions should be considered, such as education and urban heat island cooling (vegetation, white paint), thus avoiding energy-hungry air conditioning.