Sleep Without Losing the Surface
For a dolphin, sleeping deeply in the ordinary way would be risky: it must reach the surface to breathe, and it cannot simply stay underwater while its whole brain is offline. Unihemispheric slow-wave sleep offers a workaround. In this state, electrical activity associated with deep sleep appears mainly in one brain hemisphere—the left or right half—while the other remains more alert.
The alert side helps the dolphin keep track of its surroundings and carry out the basic behaviors that keep it moving and breathing. The resting side gets a period of recovery. After a while, the pattern reverses, so both hemispheres can rest in turn rather than one side being shut down all night.
An Eye That Tracks the Resting Side
The arrangement is visible in the dolphin’s eyes: one may remain open while the other closes. The open eye is generally associated with the more alert hemisphere, creating a simple link between brain state and what the animal can monitor. It is not a perfect window into everything the dolphin is thinking; it is a practical feature of sleep in an animal that must remain responsive in the water.
This split does not mean that one half of the dolphin is fully asleep and the other is fully awake in every respect. Sleep and alertness are states of brain activity, not an all-or-nothing switch. The key is that the dolphin can rest substantially while retaining enough responsiveness to surface for air and notice its surroundings.
A Tradeoff Built Around Breathing
Dolphins breathe air with lungs, but they do not breathe automatically in the same way humans do during sleep. They must surface to take a breath. Unihemispheric sleep helps them balance that need with the restorative role of sleep: one side rests, while the other remains available to support essential behavior. The open eye is more than an odd sight—it reflects a way of sleeping shaped by life in a world where every breath requires a trip to the surface.
