Dolphins have developed a fascinating adaptation to stay alert while resting, known as unihemispheric slow-wave sleep (USWS). Unlike humans, who sleep with both hemispheres of the brain simultaneously, dolphins can rest one hemisphere of their brain at a time while keeping the other hemisphere awake and alert.

Here’s how it works:

1. Unihemispheric Slow-Wave Sleep (USWS):

   • Dolphins' brains are divided into two hemispheres, each capable of independent control over sleep and wakefulness.
   • During USWS, one hemisphere enters a slow-wave sleep state, characterized by slow, high-amplitude brain waves, while the other hemisphere remains awake and alert.
   • This allows dolphins to maintain essential bodily functions such as breathing and awareness of their surroundings.

2. Breathing and Awareness:

   • Dolphins are conscious breathers, meaning they need to be somewhat aware to come to the surface for air.
   • By keeping one hemisphere of the brain awake, they can continue to surface for air regularly.
   • This adaptation also helps them stay alert to potential threats, navigate, and maintain social interactions even while partially at rest.

3. Cycle and Duration:

   • Dolphins typically cycle between resting each hemisphere, allowing both sides of the brain to get adequate rest over a 24-hour period.
   • The duration of rest for each hemisphere can vary but is usually around a few hours at a time.

4. Behavioral Indicators:

   • When resting, dolphins may swim slowly and stay near the surface, occasionally coming up for air.
   • Their eyes might give a clue to their state of rest, with one eye often closed or partially closed (corresponding to the resting hemisphere), while the other remains open.

This unique sleep pattern is crucial for dolphins, enabling them to survive and thrive in their aquatic environment, where constant vigilance is necessary for avoiding predators, navigating, and maintaining social bonds.