If you track your sleep on an Apple Watch or an Oura ring, you’ve probably wondered why some nights show great deep sleep while your REM sleep tanked after sleeping late. Humans cycle through different sleep stages every 90-110 minutes, including REM, when the brain becomes highly active while temporarily paralyzing the body in a state called REM atonia.
From an evolutionary perspective, this seems dangerous. Why would natural selection preserve a state that leaves animals so vulnerable to threats like predators? The answer may lie in how other species have adapted their sleep.
Dolphins cannot allow themselves to become completely unconscious underwater. Instead, they use unihemispheric slow-wave sleep, allowing one hemisphere to rest while the other remains alert enough to coordinate treating and monitor threats, then switching after a few hours. Cetaceans also show little or no conventional REM sleep, showing that the brain adapts its sleep structure to fit environmental demands.
The platypus shows the opposite. Despite having a relatively simple cerebral cortex, it can spend around 8 hours a day in REM-like sleep, much more than most mammals. Much of this activity involves the brainstem rather than the cortex, suggesting REM sleep may have evolved long before complex cognition and could serve more important neurological functions.
For humans, these adaptations could help researchers better understand sleep disorders. In REM sleep behavior disorder (RBD), the mechanisms preventing movement during dreams malfunction, causing people to physically act out their dreams. studying how animals regulate sleep and muscle paralysis may reveal new treatment possibilities.
Animal sleep also offers insights into localized sleep, when small regions of the human brain briefly enter sleep-like states despite overall wakefulness. This phenomenon could help explain fatigue-related errors among surgeons, pilots, and drivers, potentially improving how cognitive impairment is evaluated in medical and legal settings.
Sleep isn’t a fixed, universal neurological process. Across species, evolution has modified how different brain regions rest, remain alert, and control movement. Understanding these adaptations could help scientists develop better treatments for sleep disorders and identify the hidden effects of sleep deprivation.
Citations
Lyamin, O. I., et al. (2008). Cetacean sleep: An unusual form of mammalian sleep. Neuroscience & Biobehavioral Reviews, 32(8), 1451–1484.
Siegel, J. M. (2005). Clues to the functions of mammalian sleep. Nature, 437(7063), 1264–1271.
Siegel, J. M., et al. (1999). Sleep in the platypus. Neuroscience, 91(1), 391–400.
Vyazovskiy, V. V., et al. (2011). Local sleep in awake rats. Nature, 472(7344), 443–447.