Carl Zimmer in The New York Times:
Over the past few million years, the ancestors of modern humans became dramatically different from other primates. Our forebears began walking upright, and they lost much of their body hair; they gained precision-grip fingers and developed gigantic brains. But early humans also may have evolved a less obvious but equally important advantage: a peculiar sleep pattern. “It’s really weird, compared to other primates,” said Dr. David R. Samson, a senior research scientist at Duke University. In the journal Evolutionary Anthropology, Dr. Samson and Dr. Charles L. Nunn, an evolutionary biologist at Duke, reported that human sleep is exceptionally short and deep, a pattern that may have helped give rise to our powerful minds. Until recently, scientists knew very little about how primates sleep. To document orangutan slumber, for example, Dr. Samson once rigged up infrared cameras at the Indianapolis Zoo and stayed up each night to watch the apes nod off. By observing their movements, he tracked when the orangutans fell in and out of REM sleep, in which humans experience dreams.
… Dr. Samson and Dr. Nunn found that the time each primate species spends asleep generally corresponded to its physical size, along with other factors, such as the average number of primates in a group. The one big exception: humans. We sleep a lot less than one would predict based on the patterns seen in other primates. From time to time while sleeping, we slip into REM sleep and dream. All told, we spend about 22 percent of sleep in REM, the highest ratio of REM to total sleep in any primate, the researchers reported. Dr. Samson and Dr. Nunn have an explanation for how humans ended up sleeping so little, and so often in REM. Over tens of millions of years, they assert, changes in our ancestors’ ecology drove the evolution of new sleeping patterns. Humans increasingly have been able to achieve a good night’s sleep. A number of studies suggest that REM sleep benefits the brain. Some scientists argue that it sweeps out molecular debris, and others say it consolidates new memories into lasting impressions. But it was not easy for our monkey-like ancestors to reach REM sleep. They slept on branches, and their nights were anything but easy. As monkeys try to sleep today, they get roused by winds, tree snakes and the jostling of their fellow primates. “It’s like economy class on a plane,” said Dr. Samson. Monkeys, he believes, have to rest longer to get the benefits of REM sleep.