Fish Sleep Reveals Four Distinct States, Including Eye Movements

A team of scientists at the Max Planck Institute for Biological Cybernetics in Germany recorded the brain and eye activity of zebrafish larvae over 24 hours in their natural environment. Using advanced microscopy, they identified four different sleep stages.

This surprising discovery shows that fish, once thought to have simple sleep patterns, actually experience states with eye movements similar to mammals and birds. Previously, such phases were believed exclusive to these groups.

The study found that each sleep state follows its own circadian rhythm, with a nocturnal phase of sleep without eye movements and three diurnal phases called QEM. The most common, QEM-1, occurs during the day and features reduced brain activity and difficulty waking up.

Fish experience four sleep states, including phases with eye movements, challenging previous ideas about animal rest.

This raises new questions about the role of eye movements in fish sleep and whether they serve functions like memory reactivation or waste clearance. It also indicates that vertebrate sleep architecture has deep evolutionary roots, as similar patterns were observed in multiple Danio species.

The researchers confirmed that sleep regulation in fish depends on both internal biological clocks and light exposure. The flexibility of these phases suggests complex internal mechanisms controlling their rest.

Thanks to the transparency of larval brains, future studies can explore these sleep states in greater detail. These findings open new pathways to understanding the evolution and functions of sleep across vertebrates.

Overall, this discovery shows that sleep is more diverse and ancient than previously thought, with origins dating back to early vertebrate evolution.