Comparison of Astyanax mexicanus surface fish and Pachón cavefish

Mexican cavefish (right) lack eyes and sleep little, whereas their relatives (left) that live on the surface have eyes and sleep a lot. Credit: J. B. Jaggard et al./eLife/CC BY 4.0

Animal behaviour

Fish forgo sleep thanks to a molecule in the brain

Chemical helps blind cavefish to survive on catnaps.

Even the most sleep-deprived human has nothing on a Mexican cavefish. These blind creatures have long been scientific curiosities because of their habit of sleeping just 1.5 hours a day on average — about four times less than related, sighted populations of the species (Astyanax mexicanus) that live at the surface.

A new investigation might have found an explanation for the unusual behaviour: the fish’s brain produces excessive amounts of a molecule called hypocretin/orexin (HCRT), which regulates wakefulness and appetite. Alex Keene at Florida Atlantic University in Jupiter and his colleagues discovered that the brain of the Mexican cavefish contains almost twice as many HCRT-packed neurons as that of the surface-dwelling variety.

When the team gave the fish drugs that reduced the effects of HCRT on the brain, the cavefish slept up to three times longer than untreated fish. The finding suggests that high levels of HCRT help to keep them awake.