hunter-gatherers
Was Googling "Hunter-gatherer sleep" and came across some fascinating information:
http://www.huntercourse.com/blog/2011/05/amazing-hunter-gatherer-societies-still-in-existence/
http://discovermagazine.com/2007/dec/sleeping-like-a-hunter-gatherer/
http://www.thedailybeast.com/articles/2010/10/11/hunter-gatherer-parents-better-than-todays-moms-and-dads.html
http://www.huntercourse.com/blog/2011/05/amazing-hunter-gatherer-societies-still-in-existence/
http://discovermagazine.com/2007/dec/sleeping-like-a-hunter-gatherer/
http://www.thedailybeast.com/articles/2010/10/11/hunter-gatherer-parents-better-than-todays-moms-and-dads.html