Nick Obradovich couldn't fall asleep, again. And now he was getting grumpy.
It was October 2015, and San Diego was experiencing historically warm fall temperatures in the mid-70s. The normally cool and dry city logged its three warmest October nights on record at the time during an unprecedented heat wave. The area had experienced its warmest October on record at about 7.7 degrees above average.
Obradovich was living with his wife in a condo with no air conditioner, which isn't unusual given the typically mild weather year round. He said a lot of places don't have air conditioning, especially in more bare-bones living spaces, including ones that graduate students like himself at the time can afford to live in.
He tried coping by placing a wet towel on himself while falling asleep, but he would get too cold. He covered himself with a blanket but then would run too hot. It was like a demented version of "Goldilocks and the Three Bears" but "there was no 'just right,'" he said.
For about a week during the heat wave, he struggled to fall asleep. The sleep deprivation left him too tired to continue his daily exercise routine. He and fellow graduate students weren't able to focus on their work.
"I was grumpier. My friends were a bit grumpier. We just kind of all have dour outlook on things because we weren't sleeping as well," said Obradovich, who is now principal investigator at the Max Planck Institute for Human Development. At the time, he was studying for his PhD in political science at the University of San Diego at California.
Maybe he was annoyed from the scant slumber or just having many wet towels around, but Obradovich decided to investigate if his heat-induced cheap sleep was commonplace. He wanted to put this thought to bed, even if he couldn't do so himself.
Nights warming faster than days
His findings were nothing to sleep on: Humans are already losing shut-eye in warm environments, especially at the beginning of the night. Models predict a solid sleep will further decrease as temperatures rise, especially in lower-income and elderly communities.
In his study looking at 47,000 adults in 68 countries, Obradovich and his colleagues found a notable change in sleep duration when nighttime temperatures rose above 50 degrees (10 degrees Celsius). On nights above 86 degrees, people slept about 14 minutes less on average.
Over longer chunks of time, the loss is stark: They estimate people are already losing an average of 44 hours of sleep per year - and as the warming continues, people will be hard-pressed to find a good night's rest.
Nights have warmed faster than daytime temperatures in many places around the globe. By 2100, individuals worldwide could lose about 50 to 58 hours of sleep per year.
"Right now, we're not perfectly adapted to the climates in which we live," Obradovich said. Hotter temperatures "harm our sleep kind of across the board but that relationship increases in steepness. It becomes more significant in size the hotter the temperature gets."
Lack of sleep can make you sick
We often take sleep for granted, but not getting enough shut-eye can increase our risk for many serious health issues such as poor mental health, obesity, heart problems or even early death.
For instance, Rebecca Robbins, a physician and instructor at Harvard Medical School, said our blood pressure dips to its lowest point in the day during our sleep. But without that natural dip, people are more likely to have elevated blood pressure, which can accelerate into hypertension, heart attack or stroke.
We can even see the effects of decreased sleep each year around daylight saving time in the spring in the United States, when most people fast forward their clock an hour and may lose sleep for that night. In the week following, Robbins cited more incidents of heart attacks, car crashes and injuries at work skyrocket.
"When we're not meeting these sleep health targets, a lot of things start to go wrong," said Robbins, who was not involved in the study. "With more than just a night or two, this can become pretty problematic pretty quickly, putting stress on our vital organs, increasing risk for adverse outcomes and chronic conditions."
The ideal bedroom temperature for people to fall asleep is relatively cold - between 63 to 69 degrees. A drop in our core body temperature is essential for us falling and staying asleep because it simulates drowsiness. Our bodies primarily cool our core by sending heat to our extremities, which is why our hands and feet are sometimes warmer when we are asleep.
Obradovich and his colleagues found unusually warm temperatures had the largest effect on people's bedtime duration by delaying sleep onset. Short sleep durations were the worst during the summertime and among the elderly, probably because they have more difficulty regulating their body temperature. The team also found warmer locations experienced the most sleep loss, suggesting people's bodies haven't adapted to their geographic location.
Lower income countries are also heavily affected, which Obradovich hypothesizes could be because of a lack of air-conditioning. But he plans to investigate further.
Global warming could mean less sleep
Projections show global warming also will have the biggest sleep loss in the Middle East, southeast Asia and Australia. By the end of the 21st century, people in the warmest regions are expected to lose an additional three nights of sleep per year due to higher nighttime temperatures.
While warming trends are negatively affecting sleep, research shows the story isn't quite the same with colder temperatures. Kelton Minor, co-author of the sleep research with Obradovich, said our bodies are seemingly better at adjusting to the cold than excessive warmth.
"People appear to sleep more when it is cold outside (controlling for seasonal differences in day length, etc.)," said Minor, a postdoctoral researcher at Columbia University. "Our study suggests that people may be much better at adapting their sleep to cold temperatures than hot ones."
But dozing off for too long is also unhealthy, said Jerome Siegel, a sleep researcher at the University of California at Los Angeles. He was not involved in the study. The Centers for Disease Control and Prevention says elderly adults should get seven to nine hours of sleep.
Overall, Siegel said the study's findings are not surprising and are in line with his previous work, which showed how temperature regulates sleeping patterns dating back to hunter-gathering communities in preindustrial times.
He agreed that global warming is going to disturb people's sleep, "but you can't assume that they're not going to do anything about that."
Creating a good sleep environment
Obradovich said his team's findings could help communities or policymakers better improve the sleep environment for people, such as helping to cool bedrooms more effectively.
On an individual level, Robbins said people also need to practice generally good sleep behavior.
For instance, reducing screen time 15 to 20 minutes before falling asleep as the blue lights from cellphones or computers can emulate the sun and throw off our circadian rhythm. She suggested meditation before bed can drastically help people unwind and relax, making it easier for people to fall asleep.
Another good practice: It's important to have consistent bedtimes, she said, otherwise our body can get confused on when we're supposed to be alert or tired.
"There's still kind of the lingering belief that 'I'll sleep when I'm dead' kind of attitude towards sleep," Robbins said. "There's so much work to be done to improve our collective view of sleep."