Nighttime Temperature and Human Sleep Loss in a Changing Climate

Introduction

Sleep is vital for healthy human functioning. Yet, approximately one-third of adults report sleep difficulties, making insufficient sleep a pressing public health issue (1). Too little sleep increases susceptibility to disease and chronic illness (2, 3) and harms psychological and cognitive functioning (4, 5). Both body temperature (6, 7) and ambient temperature (8, 9) significantly influence sleep patterns.

Regular and sufficient sleep serves a crucial role in maintaining and restoring the human body. At a physiological level, sleep loss can undercut the neural consolidation of new knowledge (10), the repair of skeletal muscles (11), and the efficient removal of waste from the brain (12). Insufficient sleep may also compromise immune system functioning (13), dysregulate metabolism (14), and increase systemic inflammation in the body (15). The subsequent health impacts of too little sleep are numerous, including increased risk for cardiovascular disease (3), diabetes (2), and obesity (16). From a neuropsychiatric standpoint, acute sleep deprivation is linked to worse mood (17), and sleep problems may contribute to the development of depression (4) and suicidality (18). Moreover, restricted sleep harms cognitive performance via reductions in memory, attention, and processing speed (5). Human well-being suffers without adequate rest.

Of the factors affecting sleep, temperature plays an integral role. Normal sleep-wake cycles are governed by circadian rhythms—automatic biological processes that follow a 24-hour clock—and thermoregulation is a critical determinant of both falling asleep and staying asleep (6). As the body prepares for sleep, dilation of blood vessels in the skin facilitates heat loss, producing an important signal for sleep onset: a decrease in core body temperature. This core temperature decrease is preceded by amplification of temperature at distal sites (for example, the hands and feet). The ratio of distal to proximal skin temperature is highly predictive of sleep onset (19), suggesting that heat loss from distal skin temperature regions helps to cool the core in the evening and early morning (20). Once core body temperature drops to produce sleep onset, it remains low throughout the night and rises again shortly before awakening. By affecting circadian thermoregulation, ambient temperatures can interrupt the normal physiology of sleep (21). Previous laboratory-based studies have found that exposure to elevated temperatures can prevent core body heat shedding (9) and that poor sleep is associated with elevated core body temperature (7).

Here, we report on the effect of increases in nighttime temperatures on reported nights of insufficient sleep of 765,000 U.S. residents spanning the period of 2002 to 2011. Using these data, we examine four questions. First, have atypically high nighttime temperatures harmed individuals’ reported sleep quality? Second, do the effects of nighttime temperatures on sleep vary by season? Third, are the effects most acute among those least able to cope with anomalous nighttime heat? Finally, might nighttime warming due to climate change increase the incidence of insufficient sleep in the future?