イルカの眠り方は？脳の半分ずつ交代で眠ります

How Do Dolphins Sleep?

Every night, you close your eyes and fall asleep.

While you sleep, your body keeps breathing without your help.

Dolphins cannot sleep like that.

A dolphin is a mammal, so it needs to breathe air.

But it lives in the sea.

It breathes through a hole called a blowhole.

For a dolphin, each breath is a choice.

It must swim up to the surface and open its blowhole.

So how can it sleep without drowning?

The answer is surprising.

A dolphin sleeps with only half of its brain at a time.

One half sleeps, and the other half stays awake.

Scientists call this "unihemispheric sleep."

It means "sleep with one half of the brain."

When the right half of the brain is asleep, the left eye is closed.

The awake half controls swimming and coming up to breathe.

The open eye watches for danger.

It also helps the dolphin stay close to its group.

After about two hours, the two halves change jobs.

Now the other half of the brain gets its rest.

Scientists measured the brain waves of bottlenose dolphins.

On average, these dolphins slept for about one third of the day.

Many birds sleep in a similar way, too.

Next time you go to bed, think of the dolphins in the sea.

While one half of each brain sleeps, the other half keeps watch.