Aquatic sleep could hold the secret of REM.
Like many other animals, human beings have two types of sleep: REM (rapid eye movement) and n-REM (non-rapid eye movement.) Although it is known that the former is associated with dreaming and essential to physical and psychological health, its actual function is still a mystery.
Recently, however, a group of researcher has proposed an interesting theory based on an animal study. Seals, they found, sleep very muh like human beings when on land, experiencing the same kind of REM. But seals spend most of their time in water. And, when in water, half their brain is always awake, while the other only experiences non-REM sleep.
Based on this, it was hypothesized that REM sleep, which is known to warm the brain, functions to reverse the reduced metabolism and brain cooling that occurs in n-REM sleep. This warming could be a preparation for waking, which is consistent with the fact that human beings are much more alert when they wake up from REM sleep. With half their brain always awake, REM would then be logically unnecessary in aquatic animals.
Interestingly, this theory could help explain narcolepsy, which is characterized by a disruption of the REM sleep mechanism.
Source: Lyamin et alia (2018)