Evolution: We sleep like the fishes

Zebrafish experience similar sleep stages as mammals, birds and reptiles, according to a paper published this week in Nature. These findings suggest that sleep as humans experience it may have emerged as far back as 450 million years ago.

Sleep has been described in all branches of the animal kingdom using behavioural criteria and, in the case of humans, the main electrophysiological hallmarks have been identified: deep and rapid eye movement (REM) sleep. These sleep states have also been found in other mammals, birds and reptiles, but it has remained unclear whether fish and amphibians, earlier common ancestors of humans, experience these same states.

Philippe Mourrain and colleagues measured brain-wide activity along with eye movement, muscle dynamics and heart rate of two-week-old zebrafish larvae during sleep. Using these measurements, the authors were able to identify the first neuronal sleep signatures in this type of fish. They identified various sleep states - including slow-bursting, deep sleep and REM - coupled with characteristic muscle signatures, including in the heart and eye. The authors suggest that these findings are likely to mark ancestral sleep functions that are essential across vertebrates.