Sleeping with half the brain – is that even possible? Yes, it is. Many birds and aquatic mammals sleep with half the brain at any time, according to Scientific American (June 2019) [https://www.scientificamerican.com/magazine/sa/2019/06-01/?redirect=1]
Mammals that live in water (whales, dolphins, porpoises) might drown if they fall asleep. They cannot breathe when under water. Therefore, they sleep with one side of the brain at any one time. The other half of the brain is awake and alert. One eye is open, so the animal can make sure it stays floating and can breathe. One side of the body moves around as necessary to generate heat and keep warm. Thus, these animals do sleep. They have deep sleep (slow wave sleep), but only in one half of the brain at any time. This is unihemispheric slow wave sleep. They do not have any REM sleep (suggesting REM sleep may not be an essential part of mammalian sleep) or bihemispheric slow wave sleep.
Northern fur seals may sleep on land or in water. They sleep with both sides of the brain on land. In water, they sleep with one side of the brain, with the other side being awake. They have no REM sleep when sleeping in water.
Many birds also sleep with one half of the brain. Great frigate birds do so when migrating. They also spend very much less time asleep when migrating. However, they are able to stay alert and attentive despite this. Since their time asleep is always with half a brain, dolphins do not get much sleep. Yet, they are able to attend and learn well. This suggests biological mechanisms that may allow some animals to maintain attention and wakefulness ability, even when they need to dramatically decrease time in bed.
Mallard ducks can sleep with half the brain. This happens especially with ducks at the edge of a group. They seem to be standing guard with the outside eye open and half their brain asleep. Newborn chicks also sleep with half their brain. The half of the brain more involved in activities seems to need and get more sleep.
This is all fascinating. When we sleep, we put ourselves in possible danger. This was especially true when we lived and slept in the wild. Why did we not develop the ability to sleep with half the brain? Maybe we developed snoring to scare off potential predators and sleep apnea so we would not go into deep sleep and would arouse more easily with potential danger (just kidding)!