Caves are dark, sheltered and often quiet. They’re seemingly ideal places for a bit of a nap. But for a small Mexican fish, they have done exactly the opposite. As a result of life in dark caves, the blind cavefish has evolved sleeplessness, on at least three separate occasions. They don’t go entirely without sleep, but they doze far less than their surface-dwelling relatives.

The blind cavefish (Astyanax mexicanus) is a sightless version of a popular aquarium species, the Mexican tetra. They live in 29 deep caves scattered throughout Mexico, which their sighted ancestors colonised in the middle of the Pleistocene era. In this environment of perpetual darkness, the eyes of these forerunners were of little use and as generations passed, they disappeared entirely. Today, the fish are born with eyes that degenerate as they get older. Eventually, their useless husks are covered by skin.

They went through other changes too. For example, their skin lost its pigment so they are all pinkish-white in colour. And now, Erik Duboué from New York University had found that they also sleep less than their relatives on the surface.

Duboué works in the lab ...