Do dolphins and whales sleep

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But whales and dolphins have a voluntary respiratory system, they have to swim to the surface to breathe and then hold their breathe under water, so part of the brain must be kept alert to prompt them.

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Q.� How do whales and dolphins sleep

A.� These animals get around the problem of sleeping without drowning via a number of ways. Mainly they either rest quietly in the water, vertically or horizontally, or sleep while swimming slowly next to another animal.

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One species, the bottlenose dolphin, sleeps by shutting down one half of its brain at a time, while the other half remains awake; they even keep one eye open. The awake half of the brain is alert enough to watch out for predators and to signal when it's time to go up for air.

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Q.� How long do they sleep for

A.� This varies from species to species. Technically, they are completely asleep only while they hold their breath, or for up to five minutes.

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Killer whales, Orcas, usually sleep for stretches of 10 minutes, often together in groups. But they have been known to rest for several hours.

Bottlenose dolphins spend around a third of their day sleeping. They usually swap sides of their brains after a couple of hours to give the other side a break.

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Q.� Do whales and dolphins sleep every night

A.� Whales sleep periods occur irregularly, once or even several times a day.

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Dolphins generally sleep at night, but only for a couple hours at a time; they are often active late at night, possibly because this is the time when their food, fish and squid, rise to the surface.

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Q.� Can baby dolphins and whales sleep and swim from the start

A.� Young whales and dolphins actually rest, eat and sleep while their mother swims, towing them along in her slipstream, known as echelon swimming. This has to carry on for the first few weeks of a newborns life, if the mother stops swimming then the calf will sink as it's not born with enough body fat or blubber to float easily. At these times, the mother will also sleep on the move.

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by Lisa Cardy