| > Why that can't happen without sleep is another interesting question. I would guess that since sleep has been optimized by evolution for a long time, and evolution is so creative, that there are now countless dependencies on that time/phase separation. But umbrella reasons probably include all of these: 1. To recognize unhealthy build of material for removal, you have to stall normal production. Otherwise production and removal would waste each other's efforts and pernicious build up would be undetectable. 2. Some cellular machinery may be used during normal and clean up operations, in different things. Evolution is great at reusing whatever is available. 3. Sleep involves coordinating many systems. Some subsystems might technically be capable of operating more flexibly, but coordinating all the systems together makes overall coordination simple, efficient and reliable. 4. Sleep isn't low energy behavior. Segmenting energy use between waking and sleeping maintains a lower metabolism envelope. 5. Dreaming is high level behavior. Can't high level think and dream at the same time. 6. Dreaming sensory free, motor free, operation is fundamentally at odds with normal active sensory and motor activity. 7. A billion random dependences on phase separation have accumulated over millennia, at the psychological, gene, epigenetic and chemical level, with no pre-ordaned rationale other than the separation was dependable for the other reasons given. Intersting that octopuses, whose brains are an entirely separate "alien" evolutionary invention, also appear to dream. [0] [0] https://www.smithsonianmag.com/smart-news/octopuses-may-have... |