Dolphins' brains never fully shut off

Dolphins cannot afford to lose consciousness completely because they are conscious breathers. Unlike humans, who breathe automatically, a dolphin must actively decide when to swim to the surface for air. If they fell into a deep sleep, they would simply stop breathing and drown.

To solve this, they use unihemispheric slow-wave sleep. They shut down the left side of their brain and close the right eye for a few hours, then swap. The active half of the brain handles the swimming and keeps watch for predators while the other half gets the restorative rest it needs.

This means a dolphin's brain is never fully unconscious at any point in its life. They can continue to navigate and socialize while part of their mind is effectively dreaming. They often swim in circles or log-pool at the surface during these shifts to ensure they don't drift away from the pod.

Surprisingly, for the first few weeks of life, dolphin calves and their mothers show remarkably high activity levels with very little rest compared to adults. They stay constantly active to keep their body temperature up in the cold water, and their mothers stay awake with them, proving that the mammalian brain is capable of minimizing sleep significantly when survival is on the line.