Dhaka, Oct. 9 -- Most people know that darkness signals the brain to wind down for sleep. But a growing body of research suggests another environmental cue may be just as powerful -- and far less understood: temperature.

Scientists studying sleep in non-industrial communities have found that people tend to fall asleep not when the sun goes down but when the surrounding air cools, reports BBC.

Researchers who spent time with the Tsimane people of Bolivia's Amazon, the Hadza of northern Tanzania and the San of southern Africa found that despite having no electric lighting and spending most of their days outdoors, none of these groups timed their sleep to align neatly with light and dark cycles.

Instead, their sleep appeared to track the ...