Humans are smarter because we sleep less.

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In the new study. Dr. Samson and Dr. Nunn combined that information with studies of 19 other primate species. The researchers found wide variations in how long the animals slept. Mouse lemurs doze for seventeen hours a day, for example, while humans sleep just seven hours or so a day — “the least of any primate on the planet,” said Dr. Samson. The scientists looked for factors that could explain the differences. Compared with big mammals, for example, smaller species tend to sleep more during the day and in short bouts, perhaps because they need to eat many times a day. 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.

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