The city that never sleeps? Jack Berman/Getty

The 7 hours of sleep we typically get every night often doesn’t feel like enough. Compared with our fellow primates, which spend around 12 hours of each day slumbering, humans barely get any shut-eye.

It seems we have evolved to limit how long we sleep, and that may simply be because we have more important things to do with our time, says Charles Nunn at Duke University in North Carolina. However, the trade-off might have left us more susceptible to Alzheimer’s disease.

Nunn and his colleague David Samson collected data …