How do whales sleep?
Whales and humans sleep very differently. For humans, deep sleep means a reduction in muscle activity, blood pressure and consciousness, a drop in breathing rate and the closing of the eyes. For whales however, deep sleep generally doesn’t occur. Whales are more likely to doze and remain semi-conscious while resting at the water’s surface. They may close their eyes and their breathing rate becomes lower than usual, but they must still consciously take each breath (unlike land mammals, which can breathe without having to think about taking each breath). There is evidence though, that some species such as the Sperm whale can sleep soundly at the surface for hours at a time.
Sleeping in water is quite a challenge for a mammal that must breathe regularly! Studies conducted on captive dolphins have shown that half of their brains are resting while the other half remains awake to ensure breathing functions, which are volontary for cetaceans. As for their eyes, they kept them open, closing one or, rarely both. Dolphins living in groups keep one eye open allowing them to retain visual contact with the other dolphins in their group in order to synchronize their movements. Do whales dream? Other, more recent studies on larger whales kept in captivity (belugas and grey whales) have demonstrated episodes of sleep lasting several minutes accompanied by convulsive movements or muscular spasms.These spasms resemble the nervous tics that accompany paradoxical sleep in terrestrial mammals, which is the stage of sleep associated with dreams. However, it is imposible to draw definite conclusions concerning the existence and length of this stage of sleep in cetaceans. Rest
