[amalia (OP)]

David got in contact to send us an interesting question:
If whales and dolphins need to go to the surface of the water to breathe, my question is : How do they manage to achieve going to the surface to breathe whilst they are sleeping? Are they sub-consciously aware of their environment, thus automatically going to the surface to breathe?
Do you know the answer?

[Colin2B]

A bottlenose dolphin shuts down only half of its brain, along with the opposite eye, while the other half of the brain stays awake at a low level of alertness. The attentive side watches for predators, obstacles etc, it also signals when to rise to the surface for a breath. Every two hours, it will reverse this process, resting the active side of the brain and awaking the rested half. This pattern is often called cat-napping.
Dolphins also enter a deeper form of sleep, mostly at night, called logging because the dolphin resembles a log floating at the surface.