contributing factor mentioned https://darwininamerica.wordpress.com/2014/09/29/the-control...

More useful calories to devote to awake activities such as communication, thought, planning and etc.

mentioned in the article:

>We should think of early human camps and bands as like a snail’s shell,” he says. Groups of humans may have shared simple shelters. A fire might have kept people warm and bugs away. Some group members could sleep while others kept watch.