REM and Non-REM sleep and criticality of each
Matthew speaks about how we sleep occurs in 90 minute cycles in the brain. He says that in that cycle there are multiple stages of Non REM sleep and REM sleep that occur in sequence. He also goes onto to talk about the various benefits of Non REM sleep (Immune system, memory storage, better blood pressure control etc) and REM sleep (creativity, empathy, reproductive health etc). He speaks about why sleeping 25% less in terms of hours could actually mean getting 60-90% less of REM sleep given the way the 90 minute cycles are organized.