Researchers from the ‘Greenland Institute of Natural Resources’ have collected new data showing that whales spend inactive periods of 24-48 minutes, at a constant depth of 16-38 metres, periods only broken by the whale rising to the surface for air. The researchers believe that the whales are actually asleep when this happens. This kind of ‘drift dive’ has also been seen in humpback and sperm whales, as well as some species of seal.