The Sentinel Sleep Theory: Unweaving the biological function of REM sleep

The biological function of Rapid Eye Movement (REM) sleep remains one of neuroscience's great mysteries. In this theoretical research paper, I present the evolutionary theory that explains why REM sleep exists. I demonstrate that REM sleep functions to heighten brain alertness to significantly mitigate the high vulnerability inherent in non‑REM sleep-especially in deep sleep. Every organism with a nervous system must undergo non-REM sleep, a necessity that accompanies a negative and potentially lethal consequence: a higher risk of dying. Because non‑REM sleep substantially reduces alertness and increases the death risk, REM sleep evolved as an adaptive countermeasure, making it a necessary adaptation for any organism that must sleep. My theory-grounded in evolutionary biology and voluminous empirical evidence-provides an eclectic and far‑reaching explanatory and predictive capacity. This is because it integrates hundreds of pieces of evidence and generates numerous testable hypotheses that cross multiple scientific fields, such as genetics, phylogenetics, embryology, physiology, endocrinology, and immunology. In this paper, I discuss 452 references, most of which serve to support my theory. Based on the available evidence, all empirically testable predictions that I was able to verify were corroborated. Furthermore, the theory also resisted numerous other attempts at refutation. Here, I also challenge traditional views (e.g., REM sleep aids learning and memory), arguing that these apparent functions are byproducts rather than primary evolutionary drivers. Thus, this basic research may contribute to advancing theoretical neuroscience and the planning and execution of future REM sleep research.