If it seems like you ’ re not getting enough sleep, you ’ re not alone. People have evolved to sleep much less than chimps, baboons or any other primates studied so far, a new study finds. Charles Nunn and David Samson are evolutionary anthropologists. They study how humans have evolved to behave the way we do. Nunn works at Duke University in Durham, N. C. Samson works at the University of Toronto Mississauga in Canada. In their new study, the two compared sleep patterns in 30 different species of primates, including humans. Most species slept between nine and 15 hours daily. Humans averaged just seven hours of shut-eye. Based on lifestyle and biological factors, however, people should get 9.55 hours, Nunn and Samson calculate. Most other primates in the study typically sleep as much as the scientists predicted they should. Nunn and Samson shared their findings online February 14 in the American Journal of Physical Anthropology.

The researchers argue that two long-standing features of human life may play into our short sleep times. The first stems from when humans ’ ancestors descended from the trees to sleep on the ground. At that point, people probably had to spend more time awake to guard against predators. The second may reflect the intense pressure humans face to learn and teach new skills and to make social connections. That has left less time for sleep.

As sleep declined, rapid-eye movement — or REM — sleep took on an outsize role in humans, Nunn and Samson propose. REM sleep is when we dream. And it has been linked to learning and memory.

“ It ’ s pretty surprising that non-REM sleep time is so low in humans, ” Nunn says. “ But something had to give as we slept less. ”