Marine biologists tracking cetaceans above the tree line seem to push out puffs of air to help manage buoyancy while sleeping upright in the water.
A team attached tracking tags to several large mammals to document natural sleep behavior without disturbance.
The work hints that the oversized melon-shaped organs appear not to throw off balance during quiet moments.
Several of the cows have been seen floating inverted below the surface as a way to hold onto calories.
Posing like that could also help cows feeding calves while away from the group, members of the team point out.
These shifts in buoyancy help broaden the limited record of how giants of the deep pause for hours at a time, scientists add.