Silent sleep: risks to whales, porpoises and dolphins

Marine Biologist and researcher with Gateway Antarctica at the University of Canterbury Andrew Wright has found whales, dolphins and porpoises are putting themselves in danger when they sleep. Andrew tells Kathryn Ryan his research discovers porpoises go into a slow low-energy dive when they sleep, with half their brain switched off - the part that sends out echolocation clicks, meaning they can't detect fishing nets and may end up in them.