The Unique Human Investment in REM Sleep

We've gradually reduced NREM while conserving REM.

Posted Jan 08, 2019

Recent evidence suggests that human beings have uniquely invested in REM sleep when compared to other primates. Nunn and Samson (Nunn, C and Samson, D. (2018) Sleep in a comparative context: Investigating how human sleep differs from sleep in other primates. Am J Phys Anthropol. 2018;166: 601–612) analyzed a large dataset composed of a host of sleep, ecological, physiologic and life history characteristics of some 30 differing primate species including humans.  Using phylogenetic methods including two different Bayesian methods: phylogenetic prediction based on phylogenetic generalized least squares and a multistate Onrstein-Uhlenbeck (OU) evolutionary model of random drift and stabilizing selection, the authors were able to identify what kind of sleep characteristics a primate like us should evidence given the physiologic, ecologic and life-history characteristics we possess. It turns out that humans sleep less than predicted for a primate of our body mass, predation risk, brain size, foraging needs, sexual selection, and diet. Humans were predicted to spend 13.8% of their TST in REM. The observed value was 22.3%. Humans pack an unexpectedly higher proportion of REM sleep within a shorter overall sleep duration compared to other primates, and do so by reducing NREM sleep (rather than increasing REM). Apparently, it is easier to eliminate physiologic processes along the evolutionary path than to acquire them. At some point in our evolutionary divergence from our primate cousins we chose to reduce the total amount of time we spend in sleep and particularly NREM sleep. And conversely, we chose to conserve and actually increase by a small increment the amount of time we spend in REM. In short, we have leaned quite heavily on REM sleep in our departure from traditional primate ways. The obvious question to ask then is to what extent is REM responsible for human outlier status when it comes to intelligence or cognitive and cultural accomplishments?  The authors themselves suggest that the evolutionary reduction in NREM and investment REM produced new opportunities for learning, creating material objects, and socializing. In my view it is reasonable to suppose that this new evolutionary opportunity for cultural learning, afforded by NREM reduction and REM enhancement, helped to fuel the extraordinary advances humans made in cultural accomplishments. REM sleep is crucial to the human project.