Unihemispheric slow-wave sleep (USWS) is the ability to sleep with one half of the brain while the other half remains alert. This is in contrast to normal sleep where both eyes are shut and both halves of the brain show reduced consciousness
On land, human beings and other mammals breathe involuntarily: If we don’t make a decision to breathe or not to breathe, our body will take in air automatically. Because of their undersea environment, whales and dolphins must be conscious breathers: They have to actively decide when to breathe. Consequently, in order to breathe, they have to be conscious. This presents a problem, since mammalian brains need to enter an unconscious state from time to time in order to function correctly (see How Sleep Works to find out why this might be).
There’s plenty of time for a dolphin to catch a catnap between trips to the ocean surface, of course, but this isn’t a viable option. When you’re a conscious breather, it’s just not feasible to be completely unconscious — what if you don’t wake up in time? The solution for whales and dolphins is to let one half of the brain sleep at a time. In this way, the animal is never completely unconscious, but it still gets the rest it needs.
Most mammal species need significantly more sleep when they’re young, although baby orcas and baby bottlenose dolphins appear to not sleep at all during the first few months of life.
There are also examples of animals that can disrupt their normal sleeping patterns for certain special events, such as migratory birds which can survive with significantly less sleep during migratory seasons without building up any sleep debt. Some have been shown to take extremely brief power naps of just a few seconds, sometimes using unihemispheric sleep to remain semialert to their surroundings. During unihemispheric sleep, which is also practiced by some marine mammals like whales and dolphins, half the brain powers down into various sleepy-time modes, while the other half remains ready for action.