Sperm whales in the Arctic Circle seem to push out puffs of air in order to manage buoyancy while sleeping in a vertical stance.
Tagged animals included several large mammals as part of efforts to study resting patterns in the wild in open water.
New findings suggest the whales' large heads do not cause tipping or wobble during quiet moments.
Some females appear to rest upside down below the surface as a recovery method to save strength.
Posing like that could offer support to females raising young between feedings, researchers say.
That noted adjustment add to the limited record of cetacean idle states, the authors note, in plain language.