The group, led by Nick Obradovich, compared nighttime temperature data from 750,000 US respondents. They found a direct correlation between increases in night temperature and reports of restless, sleepless nights. The study represents the largest investigation into the relationship between sleep and ambient temperature in history, and provides strong evidence that climate change disrupts human sleep.
The largest effects were observed during summer, among both lower-income and elderly respondents. Historical estimates were combined with climate model projections, and detail the potential sleep impacts of future climatic changes. Over a single month, the researchers found a single degree Celsius increase in average temperatures produced a rise of three restless nights' sleep. By 2050, a bad night's sleep could be twice as common as now due to this effect.
Under normal conditions, human body temperature decreases from 98.9 to 97.7 degrees Fahrenheit during sleep — the ideal temperature to maintain this temperature is between sixty to seventy degrees Fahrenheit. Though many modern homes are equipped with air conditioning, individuals are still extremely susceptible to external temperature fluctuations, which can severely blight someone's ability to sleep.
The researchers added their analysis demonstrated a "robust" link between a typical nightly temperatures and insufficient sleep, most significant during the summer and among lower-income individuals and the elderly. Subjects earning under US$50,000 annually were three times more likely to suffer a bad night of sleep compared to wealthy individuals, and over-65s were affected by restless nights of sleep twice as often as younger generations.
"Moreover, across both our city-level and geographic grid cell-level forecasts, we predict every location in the US may experience an increased incidence of insufficient sleep due to nighttime warming induced by future climate change," their paper said.
Sleep deprivation can cause a variety of physiological effects, both in the short- and long-term. These deleterious impacts include increased risk of cardiovascular disease, diabetes and obesity.
From a neuropsychiatric standpoint, acute sleep deprivation is likewise linked to worse moods, and the development of depression and suicidality. Restricted sleep harms cognitive performance via reductions in memory, attention, and processing speed. These issues are exacerbated by the close ties between temperature and climate, and human health and well-being.
As the world's climate continues to warm, with average global temperatures expected to rise by 3.6 degrees Fahrenheit by 2050, other adverse impacts on the lives of humans and other species can also be expected. A bad night's sleep is but one of the many minor but seismic threats posed by climate change.
It's not just climate that significantly effects sleep. Many other factors can affect slumbering patterns, such as a slight change in one's routine or lifestyle habits. A sleeping environment is pivotal for both sleep quality and quantity, with noise looming largest. Nonetheless, an ideal sleep milieu is subjective — some cannot sleep with any sound, no matter how mild or seemingly innocuous, while others find background noises such as ticking clocks and traffic positively comforting and conducive.
Likewise, light plays a major role, influencing internal body clocks via "light sensitive" cells in eyes' retinas that inform brains whether it is day or night, and setting sleep patterns accordingly. The invention of the electric lightbulb in the late 19th century means mankind is exposed to more light at night than ever before, and has affected sleep patterns the world over. Exposure to light in the late evening tends to confuse internal clocks, leading individuals to sleep later and later. Exposure to light in the middle of the night can reset internal body clocks entirely.
Many common chemicals directly and indirectly affect sleep too, such as caffeine, alcohol, nicotine, and antihistamines, as well as prescription medications such as beta blockers, alpha blockers, and antidepressants.
Caffeine, the world's most widely used stimulant, works by temporarily blocking adenosine receptors in these specific parts of the brain — adenosine is a chemical that builds up in the brain during wakefulness, inhibiting brain cells responsible for alertness, and giving rise to sleepiness in individuals when they have been awake for many hours. As these cells cannot sense adenosine in caffeine, they maintain their activity and humans stay alert.
Caffeine also increase the likelihood of awakenings during sleep, with depending on how much is ingested, the length of time prior to sleep someone ingests caffeine, an individual's tolerance level and sleep debt, and the phase of their body clock.
Alcohol, while commonly used as a sleep aid, severely harms the quality of an imbiber's sleep — ingesting more than one or two drinks shortly before bedtime has been shown to cause increased awakenings, and in some cases insomnia, due to alcohol's arousal effects. Alcohol also tends to worsen the symptoms of sleep apnea, further disrupting sleep in people suffering from the breathing disorder.