The findings have "profound consequences" for wildlife and their ability to adapt to the climate emergency, the researchers said, and for the ability of people to cool off at night during dangerous heatwaves.
The scientists compared the rises in daytime and night-time temperatures over the 35 years up to 2017. Global heating is increasing both, but they found that over more than half of the world's land there was a difference of at least 0.25C between the day and night rises.
In two-thirds of those places, nights were warming faster than the days, particularly in Europe, west Africa, western South America and central Asia. But in some places – southern US, Mexico and the Middle East – days were warming faster.
The changes are the result of global heating causing changes to clouds. Where cloud cover increases, sunlight is blocked during the day but the clouds retain more heat and humidity at night, like a blanket.
This leads to nights getting increasingly hotter compared with days. Where cloud cover is decreasing, mostly in regions that are already dry, there is more sunlight during the day, which pushes temperatures up more rapidly.
Ecosystems have evolved as fine balances between species and many biological activities, such as feeding, occur at specific times of day. So the asymmetric changes between night and day temperatures in many parts of the world "will have profound consequences for the species inhabiting those regions and ...