When dolphins sleep, one half of their brain remains awake. This unique sleeping behavior is known as unihemispheric slow-wave sleep (USWS).
During USWS, one hemisphere of the dolphin’s brain enters a slow-wave sleep state, while the other hemisphere remains awake and alert. The awake hemisphere allows the dolphin to continue swimming, surfacing to breathe, and maintain vigilance against potential threats. Dolphins will alternate between the sleeping and waking hemispheres, ensuring that both sides of the brain receive rest.
Dolphins often swim in circles while sleeping, with the awake hemisphere controlling the opposite eye, which remains open to monitor their surroundings. For example, if the left hemisphere is awake, the right eye will remain open.
Scientists believe that this unique sleeping adaptation has evolved due to the dolphin’s aquatic lifestyle and their need to continuously surface for air. By keeping one half of the brain awake, dolphins can avoid drowning while still obtaining the necessary rest.