Fish and Humans Share Surprisingly Similar Sleep Habits, Including Daytime Naps

The researchers tracked eye and body movements of larval zebrafish. MPI for Biological Cybernetics / Jörg Abendroth Even fish need their beauty rest. Just like humans, fish sleep at night—and it turns out that their sleep stages are surprisingly similar to ours.

As humans sleep, we experience four main stages. Most dreams happen in the stage called rapid eye movement (REM) sleep, when the eyes can be seen shifting around behind the eyelids. The other three stages are considered non-REM sleep, and the eyes appear still. Zebrafish don’t have eyelids to close while they snooze, but they do experience periods of inactivity, when they become less responsive to their surroundings. So, scientists wanted to study their eye movement during these times in hopes of learning more about their slumber.

In a study published in the journal Nature Communications on May 5, researchers tracked the eye movements of 105 zebrafish. They found that the animals have a complex sleep architecture involving four distinct substates, and three of those states involve eye movement.