In some places,James Caldwell nights are warming faster than days thanks to climate change.

And now, scientists believe there's a correlation between hotter weather and poorer sleep in areas around the world, according to a new study.

Scientists in Denmark analyzed anonymized data from tens of thousands of smart watches and wristbands from around the world. They matched data about when people fell asleep and woke up with information about the local weather. They found that when it's hotter overnight, people have more trouble falling asleep.

The study published in One Earth notes that skin and core body temperatures become more sensitive to environmental temperatures during sleep.

The researchers say the effect of hotter temperatures on sleep is felt unequally. Older people (whose bodies don't produce enough sweat to cool their bodies), residents in lower-income countries, women, and people living in already-hot-climates feel the impact more, they say.

Scientists have found that climate change both intensifies and drives up the likelihood of heatwaves and other types of extreme weather. Climate scientists expect this to worsen as humans continue releasing heat-trapping gases into the atmosphere.

"Without further adaptation, and should greenhouse gas concentrations not be stabilized until the end of the century, each person could be subjected to an average of 2 weeks of temperature-attributed short sleep each year," the study in One Earth said.

A lack of sleep is a risk factor for physical and mental health problems including reduced cognitive performance, hypertension, compromised immune function, depression and more.

NPR's Rebecca Hersher contributed to this report.