DURHAM, NORTH CAROLINA—Humans evolved to have efficient sleep patterns, according to a study of data on the sleep habits of hundreds of mammals, including 21 species of primates, complied by researchers from Duke University. Modern humans sleep an average of seven hours a night, with up to 25 percent of that time in deeper stages of sleep such as REM, or rapid eye movement sleep. Other primate species need as many as 14 to 17 hours, and in some, such as mouse lemurs, mongoose lemurs, and African green monkeys, only five percent of sleep time is spent in REM sleep. “Humans are unique in having shorter, higher quality sleep,” anthropologist David Samson said in a press release. He and colleague Charlie Nunn suggest that human ancestors started getting the most out of their rest when they started sleeping on the ground, near warm fires and in protective groups. The researchers add that a good night’s sleep may have helped human ancestors cement the skills they learned during those extra hours they were awake. To read more recent evolutionary research, go to "Our Tangled Ancestery."