Whales sleep by "turning off" one half of their brain at a time while the other half remains awake. This adaptation allows them to maintain voluntary breathing and be aware of their surroundings to avoid drowning. Since whales must consciously surface to breathe air through their blowholes, one half of the brain stays alert to control breathing while the other half rests
. During this state of unihemispheric sleep, some whales swim slowly, others rest motionless near the surface, and species like sperm whales may sleep vertically just below the surface. This behavior enables them to rest without losing the ability to surface for air. Sleep periods vary by species, often lasting from several minutes to around 30 minutes at a time, and whales typically sleep near the surface to facilitate breathing
. In summary, whales avoid drowning during sleep by resting one brain hemisphere at a time, allowing them to continue breathing consciously and remain partially alert to their environment
