Sleep is necessary for the immune system and brain to regenerate and virtually every animal more complicated than an earthworm sleeps. If you go without sleep long enough it dramatically impacts your immune system and causes you to hallucinate, neither of which promotes survival.
When a ladies mouth is not moving she is asleep
or dead probably
Or buying something!
actually ,be careful...when her tongue/mouth is immobile she must be thinking what next ,planning ,scandalizing etc.
its not silly ..do you switch OFF your computer when ACU ,Mem,HDD etc warms up? similarly , your brain cells need to recoupe after 12hrs of intense control ,memory ,action,working ,execution of body functions ,metabolism...