Thinking that lack of REM sleep may be fatal, since human must learn!
So it is not a physiological limit, a drainage of neuronal resources, rather without REM there is no abstraction of thought and learning (deep learning). However, this learning is only possible by drawing on outside energy (the closed system of the individual cannot learn). This is akin to sensory deprivation, denying the person from drawing external 'learning' energy.
A little spiritual I could say, man must make contact with God every three days at least, without which he dies.
